The Medieval period runs from the end of Late Antiquity in the fourth century to the English Renaissance of the late fifteenth century.
The early portion of the Medieval period in England is dominated by Anglo-Saxons, whose language is incomprehensible to today's speakers of English. That early portion is known as the Old English period. (It is covered in a separate section of this website.) The Old English period came to an end with the Norman Invasion of 1066. Normans spoke a dialect of French later called Anglo-Norman.
Alongside Anglo-Norman, Old English developed into Middle English. Middle English is a distinct variety of English, influenced in large part by Anglo-Norman French. For example, Old English speakers did not distinguish between /f/ and /v/. Just like speakers of Modern German, OE speakers would use both sounds ([f] and [v]) for the letter . "Aefre" was pronounced [ever]. But French speakers do distinguish these two sounds. (Vouz means "you" and fou means "crazy.") After the Conquest, English people had to distinguish between, for example, veal and feel. So, new sounds, new words, new syntax—all contribute to a significant change in the English language. And to a new literature.
The Invasion put French-speaking people at the highest levels of society. Families that ruled England also ruled and held land in France. William the Conquerer was also Duke of Normandy, and the English King continued to hold that office and its lands until the thirteenth century. Only a handful of Anglo-Saxon families remained in any postions of power. In England, French was the language of education and literature. It was not an obvious choice for Chaucer to write his Canterbury Tales in English. Consequently, the High Middle Ages in England were characterized culturally by their close relation to French and Italian arts. This will change in the late thirteenth century as England and France come to loggerheads.