The Great Vowel Shift was a bit after that, actually. 1300s-1500s is the usual timeframe I've seen.
Also, the English of that time wouldn't have yet had the many, many words borrowed from Norman French, which are so characteristic of the modern English vocabulary.
Re: blah blah Up Slytherin
Also, the English of that time wouldn't have yet had the many, many words borrowed from Norman French, which are so characteristic of the modern English vocabulary.