As we all know, language in England has changed massively over the centuries.
How far back would someone be able to travel in time and still have a basic understanding of what most people are saying?
The person going back in time is around 30 years old, only speaks English, and is located in the north of England.
It's a lot more complicated than it sounds. Not just for the fact that the language changed considerably over time, but because it is highly dependent on in which circles you commune. The higher the educational background, the less trouble someone from 'modern day educated London' would have understanding early modern English (what we know as 'Shakespearean English'). However, that only accounts for anything after the year 1450, give or take. And that, again, depends on in which circles you enter.
As an example (taken from http://www.bardweb.net/language.html):
Time Period Example: The Lord's Prayer
Old English 450–1066 Fæder ure þu þe eart on heofonum; Si þin nama gehalgod to becume þin rice gewurþe ðin willa on eorðan swa swa on heofonum.
Middle English 1066–1450 Oure fadir that art in heuenes, halewid be thi name; thi kyndoom come to; be thi wille don in erthe as in heuene.
Early Modern English 1450–1690 Our father which art in heauen, hallowed be thy name. Thy kingdome come. Thy will be done, in earth, as it is in heauen.
Modern English 1690–Present: Our Father, who art in heaven, Hallowed be thy Name. Thy kingdom come. Thy will be done, On earth as it is in heaven.
And this is just in the 'mainstream English' of the time periods. Don't dare negate dialects thereof, lack of education meaning worse pronunciation thereof, and the simple fact that because of geographic isolation, many 'sub-dialect/languages' would spring up every 50 miles. This is why people from England/UK can identify where someone comes from just by talking to them, and that's these days with a 'unified education system'. How much more pronounced would this be in the time period you aim for?
Let alone the implications of the era you aim for. There's still serfdom/slavery to consider if you go back to the Medieval days, and that is also heavily dependent on which period as well. This is important to language, because if you go back to slavery days, that means the society would be importing slaves (England refused to sell their slaves outside of England, therefore there was only an influx of slaves) which means there would still be those that have odd accents, depending on how accurate you want to portray the scene.
This documentary is an interesting look at a preserved dialect. Can you understand it?
You can also look at literature as a guide post, especially lit with conversation. There are some places here in the USA where you can barely understand people, because the dialect is so specific (Virginia, parts of Louisiana). The Secret Garden set in Victorian times, has some very interesting language, and the MC at first has a bit of trouble understanding the native talk. The further back in time you go, the more likely communities will be isolated. That isolation contributes to a different manner of speech, words used that aren't standard, that kind of thing. Without a standardized school system and mass media even communities that speak the same language will have specialized dialects.
Something you might want to listen to: Beowulf. Yes, that's English. Really old English from about one thousand years ago. I think that's pretty incomprehensible for most people.
Shakespearean dialect is covered in this video at about 2:40 they start showing the difference, which isn't bad and is certainly understandable. That's about 1600. Language changed drastically during this time.
I'd say 1600 is about as far back as I'd go, maybe 1500, because that's when our language started to become more modern. I think the 1300s have more in common with the 900s and 1000s than it does with the 1500s. That's where I would cut it off, as far as comprehensibility is concerned.
Choose your time period and then research not just the language of the time, but the language in that specific place.
Well, there's a lot of dialects of English today (such as in parts of Britain and Australia, and "street" slangs common in many urban areas) that are unintelligible to the bulk of anglophones. It depends on several factors.
Once again, recall that our 30 year old Englishman (let's call him Bob) could find himself in many conversations today where he would have no idea what's going on: Technical discussions between professionals, criminal slangs, various dialects and so on. But while it would be difficult for Bob to conceal that he's an outsider, if the other person is helpful it should be rare that they can't find a way to communicate (eg. speaking more clearly, slowly, without slang or in a more neutral accent).
Up to roughly the 50s-60s, this situation remains the same. From various recordings, film, documentaries and other such primary sources, it seems like mainstream English underwent little substantial change. The accents may be a bit different, obnoxious teenagers may say "groovy" instead of "epic", but they pretty much talk in the same way. Possibly this has to do with the spread of TV, which established a sort of "common denominator" way of speaking for most of the anglosphere. After being amused by the trivial differences for a few weeks, Bob would get used to it and communicate without difficulty, and after a few months he could probably easily "pass" for a non-time traveler.
WW1-WW2 seems like a period when spoken English begins to differ perceptibly. TV had not yet began to dominate culture, so regional and subculture-specific idiosyncrasies were common.
In video, audio, and books written in an "everyday" language (either for effect or because the author was uneducated) begin to sound "quaint", and often feature expressions that seem confusing. Bob would be able to communicate with most people from Day 1, but he would frequently be confused about this or that subtlety of meaning. It would be a while before he doesn't have to say "Sorry but what do you mean by..." about commonplace words and expressions multiple times per conversation. It would be a long time before he can learn to "pass".
Written English would be much easier, as evidenced by books, pamphlets and signs from this period, except for the occasional euphemism-du-jour which has since fallen out of fashion.
The two world wars are handy delimiters, but I would say that this period was only really succeeded in the 50s (and maybe even 60s), and the preceding period ended several years before WW1 began.
From roughly the first third (give or take 50 years, depending on time, place and other factors) of the 19th century up to WW1, I think the non-trivial unintelligibility starts. Beyond just I got the gist of what you're saying but I'm not sure what exactly that one remark meant, we are getting into frequent I have no idea what the hell you just said territory for at least the first few days, especially with less educated people from lower classes.
Texts from this time are basically intelligible to modern readers, but it is clear that a fair bit of unintelligibility is lost in transcription, if you will. For instance, consider novels which try to render authentic every day speech, such as some of my Mark Twain's works.
I'd conjecture that the period is associated with the Industrial Revolution, the dominance of the British Empire, and the advent of the Prussian education system (which is ubiquitous to this day).
In early 1800s, 1700s, and maybe late 1600s we get written works (even on complex topics like philosophy, history, religion and science) that are pretty much intelligible, but sometimes effortful reading much helped by a good dictionary.
Bob could, at worst, carry around a pad and communicate by writing, assuming he can find someone literate: we are now just before public education became common, and coincidentally literacy rates are shockingly low.
Alternatively, he could attempt to speak like he would write, but there are still large differences in pronunciation, vocabulary and what is considered idiomatic. He would struggle to introduce himself to the first person he meets. If the person has some complex concerns (eg. a guardsman on the lookout for spies) I don't think he could resolve the situation diplomatically by the end of the day. If it's not weeks before he stops feeling like he doesn't even speak the same language, he should consider himself lucky.
It might be a year or more until Bob is "fluent", and 5-10 years before he can pass. Once this process is complete, if he returns to his own time, he will experience a similar culture shock for several months to a couple of years.
Early Modern English, from which our current language evolved, was prominent from 1400s to late 1600s. During this time, English went from its pre-15th century form, which is unintelligible to modern speakers, to what we have now. We conveniently have Shakespeare's works, among many others, to demonstrate the nature of EME: It is very obviously on the cusp of being an entirely foreign language in all but name.
Even written material is difficult to understand unabridged. An educated reader (even if the education includes reading Shakespeare!) can understand a fair bit of Shakespeare's writing, but there will still be a significant chunk which is hopelessly opaque: Too many words are simply not used anymore, or mean entirely different things. Without a glossary, it would be a tremendous effort to decipher them.
We have no way of knowing exactly how people spoke day to day around this time. However, it's likely very different. Unlike the period above, you now have a vicious circle sabotaging any efforts at communication: Ordinarily, you would segment speech into words and then try to figure out the unfamiliar ones, but the pronunciation is too unfamiliar for that. Yet you can't figure out the pronunciation either, since you don't know what the words are and can't effectively look for them in the sentence!
Bob is two thirds of the way to learning a whole different (but similar) language, like French. The vocabulary is different (even though a lot of words are the same, or at least about the same), the phonology is different, it's only the grammar that is mostly the same. How much does it take for Bob to become fluent in French? Let's say 6 years with instruction, well, call it 4 for EME. Of course, that is with instruction...
The key event here is probably the printing press, which began spreading around England in the 1400s. Note, also, that only towards the end of this period did English start spreading to areas of the world (America) outside of Britain proper.
We are now firmly on the other side of the hump and into "it might as well be a foreign country" area.
A good, familiar example of this is Chaucer's writing. Bob can't read it. He literally can't even read the script. If it was put into a modern font (something impossible without professional help), he would not understand the words. If it was spoken it would sound like gibberish.
Considering the various social aspects of this period, I would seriously doubt Bob's long-term survival. He could easily get killed, imprisoned, or otherwise get into a lot of trouble due to inevitable misunderstandings and miscommunications.
If you have Bob ever managing to communicate with anyone, such as after years of trial and error, you have to invoke a stroke of luck soon after his arrival, such as a kindly benefactor who supports him for many months when he is unable to function in society.
The key event is the Norman Invasion in 1066, which is when a bunch of Frenchmen invaded Britain, put themselves in charge, and didn't even bother speaking proper English.
Remember how I compared learning early English to learning French? If Bob arrives prior to the Norman Conquest, it might actually be harder. I would say that difficulty is comparable to learning a Nordic language, such as Norwegian, today. I doubt Bob would do any better than Hans or Xavier if he went this far back.
The key event here is the Roman conquest of Britain, and the spread of Christianity: Both provided a host of Latin words and influence, which produces a fair bit of common ground for modern speakers to discover.
However, if you go farther than that, just give up on having any intelligibility whatsoever. As far as 500 BC, I suspect it won't make much difference to Bob whether he arrives in the British Isles or China. Good luck, Bob! Wouldn't wanna be in your shoes!
NB: The periods I give above are very approximate. I tried to note what historical events strongly influence the change in language, but ultimately different parts of the world were undergoing the change at different rates, as were different strata of society.
1066~1440 CE
before the normans conquered england in 1066-1072ce, the residents of england spoke a language called "old english", which is arguably more similar to modern german than modern english (e.g. Beowulf 700~1050ce). so, before 1066 you would have almost no chance of understanding anyone without significant study. after the normans conquered england, the conqueror's "old french" mixed with the commoner's "old english" to form a new simpler language we today call "english". that said, it took a few generations for the languages to mix, so in 1073 you would be just about as lost as 1065, but by 1300's things would start sounding familiar (e.g. the canterbury tales from 1386). still, without modern printing, dialects varied widely between towns, classes and generations. in fact, a typical uneducated serf would have a gruff german vocabulary you would consider rudimentary, while an "old money" noble conversation would sound closer to french and might actually include a few french or latin words. it wasn't until the printing press was invented in 1440 that the english language started to unify and stabilize into what we would recognize as "modern english" (e.g. asop's fables from 1484, typeface notwithstanding). so, listening to an educated person speak in 1450 london would be almost perfectly intelligible. you might need a few days to get used to their accent, word choice and world view, but the vocabulary and grammar would be perfectly serviceable to discuss most topics.
futher reading: https://en.wikipedia.org/wiki/Middle_English_creole_hypothesis
side notes:
you must keep in mind that the culture of pre-modern england was very different from today. on the one hand, without mass transportation, most people lived their entire lives within 40km of their home. with no dictionaries, they expected to have difficulty communicating with anyone who lived more than one village away. as such, they would probably be more adept at bridging the dialect gap than you. in a big city like london, you could probably find someone you could understand quickly. on the other hand, rural strangercide was much more common, and they might just kill you rather than try to figure out why you left your previous home (presumably not by choice...). at the very least, once the local lord determined you did not belong to any other lord, he would probably try to force you to work his land rather than accept the preposterous premise that your knowledge could be more valuable than your muscles.
even today, you can see the upper/lower class french/german split in distinctions like a restaurant's "beef" (french bœuf) vs a farmer's "cow" (german kuh). or lower class german expressions like "blow up" vs upper class terminology such as "explode".
To answer in a more simple way than the other answers which are all fairly good... There are places today that if I dropped you in the middle of as an English speaker would have no clue what the other party is saying even though you are both speaking English, such as the stereotypical Scotsman, or someone using Cockney rhyming slang. So the answer to your question is 0 far back and just move you to a different place.
As far as going back as far as possibly till you can't understand anymore. That's highly variable and depends on what is being said and how good your character is at figuring things out.
"What's up, my brother?"
"Wassup, my brudda?"
These are modern english and understandable.
Changes happen at a rough pace of 1 sweeping change every 30 years it seems to me, so let's say we move you 100 years in the future...
"Wassub, my brotta?"
"Wazzup, my blutta?"
You can still understand that, but what if used another variation... "'Sup, m'brother?"
in 100 years using the same alterations as before...
"Sub, m'brotta?"
"Zup, m'blutta?"
Do you think you'd be able to understand that? But I'm sure you'd understand..
"This is m'brotta n sista"
"Thiz iz m'blutta n zizta"
Now if you went for 500 years, it is likely that any changes that happened in that 500 years isn't going to change again. For example s => z happens so that z seems to be safe till the next 500 year cycle. So really the question is when you apply around 15 changes to the most common words in the language how long is it before you haven't got a clue? I imagine 3 or 4 jumps before they mostly look like a scrambled mess. So I'd place it around the 500 CE mark where things start becoming fully incomprehensible, but for the average person I'd expect 1500 CE, maybe a little bit further back. Just read Shakespeare as it was written and see how much trouble you have understanding it. That was written for the masses, so if you can't understand it, you're likely not able to understand 500 years ago.
It is going to depend primarily on the size of the person's vocabulary and their ear for accents. Go read some snippets of Shakespeare and Chaucer.
Someone who is familiar with the second, third, and fourth meanings of the words they know, and who has a large vocabulary will have the ability to understand anything later than Old English with a little effort. The words in common usage have changed, but are still largely recognized with their original meanings. The pronunciations have changed a bit over the centuries as well, but in a more-or-less regular pattern, so it's not much worse than understanding someone with a heavy accent. Read some of Shakespeare's sonnets and soliloquies, keeping in mind that they many are supposed to rhyme and have a particular syllable stress pattern. You can use the rhyme and pattern to figure out how the original performer would have pronounced the words. (What's more, you'll notice that the different characters have different accents depending on social class and origin; Shakespeare really was a master playwright, but I digress.) Mastering the local idiom will take some effort, but that's true even of modern English. Slang has a lot of variation from place to place.
Once it switches over to Old English, you're in an entirely different boat. Grab a copy of Beowulf and a pronunciation guide and try reading some of it aloud. Some words are recognizable, particularly the ones for important concepts. But the common pronouns and tenses are almost all different. The last time I tried to sort it out, I couldn't recognize more than about 20% of the words and phrases, and that was with a literal translation available to make some of the patterns more clear. The language is similar enough for total immersion to probably cut the learning time for a well-educated speaker down to a few months instead of a few years, but that's about all you get.
On the other end of the spectrum, the average "publik skool gradyouate" these days probably doesn't have a sufficiently large vocabulary to hold an intelligent conversation with anyone from more than about 150 years ago at best. The words are all still English, but the most commonly used terms for things has shifted radically in some areas. "Gramophone" instead of "record player" for example. How many are a "score"? What body part is the "gob"? A lot of scientific terms had yet to be standardized, as did a lot of word spellings. You can find old journals, diaries and political speeches on the web to use for comparison.
As a starting point: there are still English accents around even today, inside England, with which some native English speakers have a fair bit of difficulty. This is alleviated by "received pronunciation" a.k.a "BBC English" or "Home counties English". Basically, everyone listens to the TV and radio, and learns to understand that accent, and to a greater or lesser extent to shift their pronunciation towards RP if they are talking to someone from way out of town. Also drop any dialect words in favour of the standard ones, in the same way that I will substitute sidewalk for pavement and so on when I'm in the USA. (This is assuming a desire to converse ... a good way of persuading strangers to go away is to remain resolutely incomprehensible! )
It seems to be quite idiosyncratic as to who has trouble with which accent. Personally, I find Glaswegian the hardest. (OK, Glasgow is in Scotland, but the vocabulary is English, and other Scots don't baffle me as long as they're not speaking Gaelic). A friend has real trouble with Devon accents. I have none at all.
Anyway, go back a couple of centuries, and there was no Radio and TV, so no nationally standardized received pronunciation, and no railways so visitors from a couple of hundred miles away were rare. I expect that this resulted in a greater diversity of regional variations. So I expect that at first hearing, you might be unable to communicate. On the other hand, again assuming a desire to converse, you would probably recognise that you were hearing English spoken with a strong accent (as opposed to, say, Dutch), and adjust, and learn quite rapidly. As a recent example of this process, a relative (from London) went to University in Liverpool where she shared a flat with some locals. After a term away, her accent had shifted very considerably North-West, and when she came back home it took a week or so of hilarity before her London accent reasserted itself.
The question is unanswerable: "travel in time and have a basic understanding of what most people are saying". We can see that Shakespeare's English has vocabulary and grammar that is fairly accessible to a modern reader of English, and that Chaucer is much less so. However, we cannot listen to English as was spoken in a particular time and place centuries ago. I expect three of four centuries back in London would be far more challenging than three or four hundred miles into a rural community with a strong local accent today. I also suspect that it would be far less of a mountain to climb, than trying to learn Dutch without any textbooks or conveniently bilingual Dutchmen. (Dutch chosen because it's probably closer to the Anglo-Saxon roots of English than other European languages.)
Edit: RP itself evolves quite fast enough to notice and grate upon those more than half a century old. Recently seen in "the Telegraph", a letter requesting that BBC commentators cease pronouncing athletes as ath-uh-letes I hadn't noticed, until I read that, and now it's also grating on me!
A key point was the advent of printing, especially printing with movable type. This made text a lot cheaper to produce and so there was more of an incentive for people to become literate which naturally tends to standardise language as texts are disseminated more widely.
At this point it is also worth noting that through much of the period when English was developing Latin and Greek were the standard languages for academia and theology. Note that Isaac Newton's Principia, one of the first texts to be generally recognised as science in the modern sense was originally published in Latin.
Similarity the use of Latin rather than 'Vulgate' languages (such as English) was one of the major issues of the Reformation, note Martin Luther's translation of the Bible into German in 1522 (with Tyndale's translation into English not long afterwards).
Religion was important because it was the one universal context in which people at this time had with the written word (albeit mediated by priests) and translating the Bible and Church liturgy into English meant that it was something which people could understand rather than just being exotic mystical words.
The Anglican Book of Common Prayer, printed in English and widely distributed is also widely accredited as a major milestone in the establishment of modern English as it Dr Johnson's famous Dictionary.
As others have mentioned a crucial aspect of this is that standardisation was hugely important in establishing modern English as a distinct language and before that there were many regional dialects which were often hybrids of several layers of languages from various periods of settlement and conquest, which can be seen from place names.
Even now there are areas, especially in the north east, west midlands and Scotland where there is still a residue of very strong regional dialects, which go much further than just accent or slang, which can be virtually incomprehensible at times to non-locals.
So to directly address the question it seems fair to say that by around the early 1600's comprehensible modern English was well on its way, for example the works of Shakespeare are mostly comprehensible to modern speakers, especially considering that he was writing in quite a stylised form and most of the differences are vocabulary rather than grammar.
So we can say with some confidence that by the 1600s a recognisable form of modern English was in reasonably wide circulation. Indeed Shakespeare himself was a middle class midlander rather than part of the academic, political or religious elite.
Going back much further than that we encounter the problem that we have less evident to go on as written or printed texts tend not to reflect a wide section of the populous. We can say though that works like the Canterbury Tales (1386) are pretty hard going, especially with the original spelling. Le Morte d'Arthur(1485) is interesting in this context. The original is clearly significantly different from modern English but this is mostly about how spelling is rendered phonetically and versions with modern spelling are reasonably comprehensible in terms of sense, although the writing style is incredibly dense and very hard work to work out what is going on with the story compared to a modern play or novel.
By contrast if you look at something like Beowulf, the Eddas or Icelandic sagas, especially with an inline translation the way that the language actually functions in terms of sound, rhythm and conveying meaning isn't all that alien to a native English speaker, especially if you compare it to documents directly translated from say Chinese.
