Partielo | How English was formed ?

It all started with Old English formed by the mix of the two dialects of the Angles and the Saxons, two Danemark tribes. It slowly replaces Celtic. It is incomprehensible to us modern English speakers. As an example the Poem of Beowulf was written in Old English.
The first major change of the Old English happened around the 8th century when Vikings invaded England and added words to the vocabulary.
In 1066 William the Bastard (and later on the Conqueror) successfully won the battle of Hastings and invaded England with his Norman army. The Norman became the ruling class and spoke a dialect of old french. The English were still speaking Old English. After some years the two languages merged and formed : Middle English. That is why we have common words in french and English today. An example : The Canterbury tales by Chaucer is written in Middle English !
In the 15th century the Great Vowel Shift happened beginning to transform the language in its "early modern" form : Early Modern English