Pronunciation of English words differ from one English speaking country to another. Pronunciation of English words also change within a country ie: England, Canadá, the United Stares.
Great Vowel Shift refers to a series of changes of the English language Between 1409 and 1799. Otto Jesperson, a Danish linguist and Anglist (a person who studies the English language or literature composed in English) is credited to the term Great Vowel Shift.
To help understand the shift in vowel pronunciation, I am including a chart from the Wikipedia website:
For additional reading, please go to:
Aloha -- Cathi