Effects of the Norman Conquest.
For centuries after the Norman Conquest in 1066, all high ranking members of society spoke a language known as Anglo-Norman, an ancester of modern French. It was also normal for lower ranking officials to be bilingual between Anglo-norman and English, while english was the only language spoken by commoners. English was strongly influenced by Anglo-norman, and later by its close relative Anglo-French. During this time French was the official language of England, but would only stay that way until roughly the 14th century.
Changes in grammar and vocabulary.
At the same time English was beginning to transfer from a synthetic language to an analytical one due to the diminishing of grammatical endings and a new focus on word order.The original plural marker '-en' also began to be replaced with the new marker '-s'. Roughly 10,000 French and Norman words entered the language in this time period, primarily words related to government, the church and the law.