After defeating Harold of Hastings and killing nearly every Saxon lord, William the Conqueror became king of England in 1066. Since the Norman lords filled the court, as well, French was the spoken and official written language for nearly 300 years. The great legends of King Arthur were written in French. Even Geoffrey Chaucer composed French verse; however, when he decided to write his Canterbury Tales in the vernacular, Old English began to dominate the written word.
Nevertheless, to this day, nearly 60% of the words in the English language are derived from the Old French, because when the Normans were all the landowners they found it necessary to communicate with the Saxon overseers; in addition much literature was written in French. Thus, a blending of French with the Saxon affected the evolution of modern English.
When the Normans conquered England, French became the prestige language of England. Because of this, many French words came to be borrowed into English. This was especially true in areas such as government and the military. Many words in these areas (governor, lieutenant, even enemy) are French words. This totally changed the vocabulary of the English language.