@kkfengdao,
Strictly speaking, there's no such thing as Medieval English. English goes through three phases: Old English, Middle English and Modern English. It's possible to also refer to early and late versions of those, but let's not complicate the issue.
Old English, also referred to as Anglo-Saxon, was the
lingua franca of several Germanic tribes who had settled on the east coast of the main island of the British Isles beginning in about the late 5th century. The primary groups were the so-called Angles (they still exist, but they call themselves Angeln, and they come from a strip of land between Germany and Denmark), the Saxons (who still exist, and once occupied a broad swath of what is now Germany), the Jutes (no longer a separate group, they've been absorbed by the Danes) and the Danes. Further adding to English were the Frisians, who also still exist, and who were the the principle sea-borne traders of the North Sea.
All of these groups contributed to the formation of Old English. A contemporary English speaker probably couldn't understand a modern Frisian speaker, but could read what they wrote--we get our syntax and a fair amount of vocabulary from them. A contemporary English speaker couldn't read what an Angeln wrote, but many of the simpler things they say would sound startlingly like modern English. All the groups made their contibutions, but it was a bewildering mish-mash of at least five different Gemanic dialects--Anglic, Saxon, Danish, Jutish and Frisian. It was horribly complex, and it tended to have strong regional dialects--and it was illiterate.
When christian missionaries came along (the original christians of the island had been driven back to the west along with everyone else among the former inhabitants) and converted these people, they began to write down the language, and Anglo-Saxon, or Old English was formed. It was still a heavily inflected and difficult langauge, but writing it down was progress as that lent a stability of spelling and usage.
In the middle of the 11th century, the Norman-French invaded England. This began a process which had a powerful influence on English. English was no longer the official language of the rulers of the island. Fewer things were written in Old English, and the language survived as a spoken rather than a written language. It was considerably simplified, too, because to an illiterate speaker of the language, inflections hardly matter--they don't care that past participles are formed by adding -en to the basic present indicative form, they just "instinctively" do it. (That is no longer true, of course, but we still have verb forms such as gotten and forgotten which retain the ancient practice. It also survives in adjectives formed from the older past participle forms, such as beaten, used to described some metalurgical effects.)
There is a document called the Peterborough Chronicle in which monks at a monastery recorded what they considered the important day-to-day events in their part of England. It is a part of the Anglo-Saxon Chronicle. There was a fire which destroyed their original manuscript, so they copied one from another monastery, but two long passages of their own succeed it. These are the first continuation and the second continuation. The first continuation, in the early 12th century, is a mix of Old English and Middle English. The second continuation appears to have been written by one author, and is in middle English. The nearly two dozen inflections were gone, nouns no longer necessarily had a gender, verb forms were much simplified--the written language more resembled the spoken langague, which had long ago abandoned many of the complexities.
Obviously, a pack of monks didn't invent middle English in a couple of generations. Middle English formed in the period between the Norman conquest (1066) and the estalishement of the Angevin dyasty (1154). (For reasons no one knows, the Peterborough chronicle falls silent after 1154.) Probably because it had stopped being the official language, the literate Anglo-Saxons--usually the monks, since French-speaking clerics, from which we get the word clerk, had become the standard of literacy with the Norman French--began to write the language in a simpler form, as it was spoken by the people. The inflections of nouns, adjectives and verbs were simplified and eventually abandoned. Regional dialects still have a large influence, but now English was in a form that, with some help, a modern English speaker could read.
So, roughly speaking, Old English dates from about the late 5th century or early 6th century (hard to say with a mostly illiterate people) to the 11th century, and middle English forms in the period from the mid-11th century to the mid-12th century, when Old English disappears in a written form.
As stife between the French and the English grew, and the Hundred Years War began, the French-speaking Normans of England began more and more to speak English, especially as the French tended to ridicule the way they spoke French. In the crucible of that long, long war (it lasted well over one hundred years, although the actually fighting only lasted about 30 years or so, at different intervals), modern English began to form as the Norman aristocracy began to make English their day to day langauge, and more and more documents, official documents, were written in English. One of the earliest writers in modern English, Geoffrey Chaucer, was a sometime diplomat for the King at the beginning of the Hundred Years War.
William Caxton was mentioned in your opening post. When printing became general in western Europe, he learned the printing trade, and printed or caused to be printed several books at Bruges in the Low Countries, written in the English of the day, and intended for sale in England. Soon he went back to England himself, and set up a printing press in London. The variety of English spoken in East Anglia and in Essex and that spoken in London were very similar, and many popular books (such as those written by Chaucer) which had previously been copied by hand were now being printed. Caxton churned out those books and several others which he himself translated from French. Other Englishmen hurried to exploit the market and set up presses of their own. The buying public then were the aristocracy, all of whom spoke the dialect of Essex/London because London had become the seat of government, even if they also spoke the dialect of the region where they lived. That version of English became the standard of modern English in the period of Early Modern English.
So Middle English lasts roughly from the mid-12th century to the late 14th century, and by the late 15th century, Early Modern English begins to take over. None of these dates are hard and fast. Essex is a region east of London, and north of the Thames River. Kent is the region south and east of London, and south of the Thames River. In the late 14th or early 15th century, a group of sailors and soldiers on their way to France landed in Kent and tried to buy eggs from a woman there (i believe it is Chaucer who recorded this incident), but failed because they couldn't make themselves understood. To them, a hen lays an egg, and to them, the plural of egg was egges (also sometimes written eggys). But to the woman in Kent, a hen lays an eye, and the plural is eyen (or eyin). So she didn't know what they hell they were talking about, and they didn't know how to make themselves understood. So even as Early Modern English was forming, some people were still using Old English forms. Language is a messy process, and what is important is communication, not adherence to someone's rules (whether that someone considers himself to be a "descriptivist" or not is irrelevant, too). The Kentish woman was a part of a language community which was starting to die out (the langauge, not the people), and literacy and the printing of books just hastened the process.
For general terms, you can think of it as Old English until the 12th century, Middle English until the 14th century, and Modern English from the 15th century to the present.
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