What is know as Anglo-Saxon, or Old English, was developed from diverse contributions of many different peoples: the Jutes from Denmark, and the Saxons from Germany. The Danes made substantial contributions, such as incorporating many words into the vocabulary and the introduction of pronouns, filling a real linguistic need. Then, the Danes simplified the language by eliminating the practice called inflection; that is, the use of word endings rather than word order for meaning.
The change from Old English to Middle English can be traced to one riveting event--the Norman Conquest of 1066. The Normans were a people descended from Vikings who had invaded the coast of France in the ninth century. Over time, they adopted many of the French ways such as becoming Christians, speaking a dialect of the French language, and organizing themselves according the French political and economic system of the times; namely, feudalism.
The primary effects of the Conquest on Old English (Anglo-Saxon) literature were that the disruption of the Norman's ended alliterative verse altogether except for in isolated regions and it buried Beowulf entirely until the 1700s. In other words, Old English alliterative verse style was all but lost and the legend of Beowulf nearly lost.
The Norman Conquest disrupted the literary culture of Anglo-Saxon England. The practice of alliterative verse continued until the fifteenth century, primarily in the north- and southwest corners [but] Beowulf disappeared from English literature until ... the eighteenth century.... (Norton Anthology of English Literature)
Having family ties to Edward the Confessor, the English king, William, Duke of Normandy, After Edward died, the Saxon witan, or council of elders, chose Harold of Hastings their king. However, William of Normandy claimed that Edward had promised him the throne. Consequently, he led a few thousand troops across the English Channel on a stormy night and claimed his throne after a bloody battle that left nearly all the English lords of the land dead.
Having slain Harold II, William claimed the throne in London where he crushed any resistance. He was crowned King William I (William "the Conqueror"). Of course, after this the Normans felt themselves vastly superior to the people they had conquered. Thus, French became the language of the court and Anglo-Saxon was relegated to the farm, and official documents were written in French. The legends of King Arthur were written in French originally and later put into English when Sir Thomas translated from French sources and composed his romance, La Morte d'Arthur in twenty-one volumes.
While Norman French was the language of government, the Latin of the Catholic Church was used for religious documents, and for literature. Gradually, the use of the vernacular increased and after 1380 John Wyclliffe began directing the English translation of the Bible. So, both Norman French and the Anglo-Saxon vernacular (Old English) contributed to what is known as Middle English. At this same time, Geoffrey Chaucer, who himself wrote French poetry, composed The Canterbury Tales in the English vernacular. Historian Edward Bowen wrote,
The poetry of Chaucer, along with other writers of the era, is credited with helping to standardise the London Dialect of the Middle English language....