English evolved from Anglo-Saxon (aka Old English), which is mutually incomprehensible with Modern English and was a very different language (5 grammatical cases, three grammatical genders, multiple words for YOU, etc).
As charu noted, English was heavily influenced by Old French vocabulary for about 300 years after the Norman conquest (grammar was very mildly affected. English still has very strong yet simplified Germanic vocabulary) - which led to Middle English, which eventually evolved into Modern English.
Anglo-Saxon evolved from previous dialects, such as proto-West-Germanic, which evolved from Proto-Germanic, which ultimately traces itself back to Proto-Indo-European.
English was not invented; it evolved; so there is no precise date for the start of English or any of its stages. You will find dates in linguistic articles, but those are approximations and are based on surviving documents (there very well could have been other documents that would change those dates, if they had not been lost).
Note: from Proto-Indo-European all the way to Modern English, each generation understood the one before and after it. Only by comparing point along the timeline that are widely separated from each other do you realize that somehow and some time the language had evolved into a new one, but there is no precise moment when you can say that Proto-Germanic or English began.
Note also: English is a Germanic language, meaning it evolved from Proto-Germanic. German is also Germanic. English did not evolve from German, though (a common misconception).
studied linguistics and the history of English and other languages