Around 450 AD, when the Roman troops withdrew from Britain, the Angles
(hence the terms English and england), Frisians, Saxons, and Jutes began
to travel from Germany and Northern Europe to Britain.
The Saxons were
the most dominant group. The Anglo-Saxon languages developed into
English. They were functionally illiterate so little is known about the
time but in 597 Christian missionaries came and introduced literacy and
Christianity to Britain.
In 1066, the Normans, Viking who had settled in France 200 years before,
took over England. They contributed 10,000 words to the English
language. English was considered a peasant language and anyone of a high
status spoke French.
As the Normans became more isolated from Europe,
they began to consider themselves English rather than French. At this
time, There were many different dialects of English and often people
living in neighbouring towns could not understand each other. English
was very casual and there were few rules. They became more regularized
through time and word forms were generally reduced.
No one can say at which point English became a separate language from
the different Germanic dialects but it would have changed quickly as
people of different tribes tried to communicate with each other. The
waves of new languages that spread over the British Isles greatly
broadened the English vocabulary. This is why we have so many homonyms
and synonyms, compared to other languages.