The Norman Conquest in 1066 was a very important event, not only for England but also for the English language.

For the next three hundred years, the Norman-French patois (what we know as 'French' didn't exist at the time) was the language used by the ruling elite of England. However, the Norman-French patois did not drift down to the English people, as the English continued to speak their local dialects. Likewise, a national English language did not exist in England.

The Hundred Years War from 1337 to 1453 was a series of wars between England and France. These wars were fought only in France. Initially, it went well for England but as time went on, the French pushed the English out of France.

From the 14th century, Norman-French went on a quick decline as the ruling classes finally started to adopt English. From the 15th century, the English turned their eyes from Europe to the British Isles and over the course of the next few centuries Ireland and Scotland were conquered, with Wales already being conquered in the 13th century.

What is incredible about these three hundred years is the amount of words which English absorbed from the Norman-French patois. A massive amount of English vocabulary comes from Norman-French. One example is with legal terms.

In the 18th and 19th centuries only the educated and upper classes would have learnt French, but in no way was French used in the 18th and 19th centuries like it was by the Normans in the first 300 years after 1066.

References:

The Mother Tongue (ISBN: 0380715430) by Bill Bryson.

HISTORY OF ENGLISH LANGUAGE 2 English Goes Underground, https://www.youtube.com/watch?v=QFyPiFJOeUs