Middle English

In 1066 AD, the King of England, Edward the Confessor, died and William Duke of Normandy decided that this was his chance to take the throne. He invaded England and crowned himself King in the battle of Hastings. However, William only spoke French and as a result of this, although the lower classes continued to speak English, the upper classes started to speak French.

This did not last very long however and in 1204 AD, the then King of England, King John lost Normandy to the French and after this English was reintroduced into the upper classes. To reinforce this use of the language, foreigners started to come to England which made the upper classes somewhat protective of their language which led to the reintroduction of the English language to the upper classes. Although they learnt and spoke English, many still used French words when speaking.

The Hundred Years war deepened the French-English divide and made French an even less desirable language for the English to speak. This was a significant period in the evolution of the English language. English was being more widely spoken, but it was being spoken by people who had previously spoken French. This meant that many of the words changed and evolved, adapted from the French.

Grammar rules and spelling can be found at: http://homes.chass.utoronto.ca/~cpercy/courses/MESpelling.htm Here you can see the French influences in some of the spelling changes.

This Middle English, as it became known, is the language that the poet Chaucer wrote his famous Canterbury Tales in, although it still very different from the language we speak today.
Sources: http://www.englishclub.com/english-language-history.htm