NORMAN INVASION OF 1066
When Guillaume (William) of Normandy conquered the England, he slaughtered the old English nobility and replaced them by his own loyal, French-speaking earls. 'Englisc' also became a pejorative for a time and all important issues were being solved in French or Latin, the English language was completely excluded from the formal matters, which caused a great influence of French on English. Thenceforth, we refer to the language as Middle/Medieval English. The biggest change, however, took place a few centuries later, by means of the so-called Great Vowel Shift.

STANDARDISATION
Because of the differences between the dialects of English, came the need to establish a certain standardised form, inasmuch as even personal pronouns could be found written in as many as 20 ways. This happened mainly due to the introduction of the movable print to England. We call this stage already Modern English or Early Modern English. The classical Germanic case system was already lost and the only case system in modern English is found in pronouns, which have three cases: subjective, objective and possessive (I, me and mine). The vast majority of the inflexion was lost as well.

GLOBALISATION
As the British empire expanded and took up the area of more that 36 sq kilometres, it was only necessary that it gained a lot of new vocabulary from the languages of the conquered. Thus a lot of Arabic, Hindi, Persian, native American and other words enterred the language.


NOTES:
I was using a term in my essay, that might be unfamiliar to some of you.
High German language is just another name for German language that is widely lectured here in Slovakia. The 'High' addition is used to distinguished this 'mountainous' language (coming from the mountainous area of present-day Germany) from the Low German language, which is not a dialect thereof, but a completely different language. Low German is descendant of Old Saxon, whilst German is descentant of Old High German.

I might edit this essay from time to time, to correct some possible errors, to include more information and to clarify some issues discussed here.