After the William the Duke of Normandy conquered England in 1066, the aristocracy was dominated by French speakers. Although those at the top spoke French, the peasantry of the country remained English-speakers. Although the French-speakers and English-speakers remained separate at first, over time the two languages began to merge due to either intermarriage, business, or simply interactions between the two. English was "demoted to to everyday, unprestigious uses," while French was the language of "the court, administration, and culture" (Lawless). During the occupation over 10,000 French words were added to the English language. Out of these 10,000 words, about three fourths of them are still used in Modern English. It's estimated that the average English speaker "who has never studied French already know 15,000 French words" (Lawless). In fact, there are about 1,700 identical cognate words between the two languages. The incorporation of French into the English language also contributed to one of English language's large amount of synonyms.
Sources:
http://french.about.com/od/vocabulary/a/frenchinenglish.htm
http://en.wikipedia.org/wiki/English_language
