The Great Vowel Shift.
The Great Vowel Shift occured in the 15th century, and is said to have established modern English. The shift consisted of the seven original vowels from Old and Middle English shifting up one position in the mouth to become a higher sound. Two of the seven vowel sounds were too high to move any further in the mouth, and so instead became the dipthongs /aʊ/ and /aɪ/. This shift among other things is the diference that makes middle english sound foreign and virtually unrecognisable as English.
Foreign Influences and Standardisation.
An increase in literacy and travel around this time assisted in English adopting words from more and more foreign languages, including Italian, German and Yiddish, as well as even more influences from Latin and Greek. The first full featured dictionary of the English language (titled 'Dictionary of the English language) was published in 1755 and only helped to further standardise english and solidify both the definitions of words and the contexts in which they can be used. The language has only evolved more and more since the first dictionary to be one of the most influential langauges world wide, with its speakers estimated to be around one billion.