Many English speaking students of other languages must be confused when confronted with the long vowels of a language such as French. "Why do these foreigners all say their vowels wrong?" they must think. The French long "i", for example, is pronounced like the English long "e". Why is this so? The answer is the almost rudely named phenomenon of The Great Vowel Shift of English pronunciation.
What Was It?
The Great Vowel Shift was a gradual process which began in Chaucer's time (early fifteenth century) and was continuing through the time of Shakespeare (early seventeenth century).
Speakers of English gradually changed the parts of their mouth used to articulate the long vowels. Simply put, the articulation point moved upward in the mouth. The vowels which began being pronounced at the top could not be moved farther up (without poking into the nose); they became diphthongs
The upshot has been that the Anglo-Saxons lived (like the Scottish still do) in a hoose, and we live in a house; the Anglo-Saxons (like the Scottish -- hmm) milked a coo, and we milk a cow; an Anglo-Saxon had a gode day and we have a good one; an Anglo-Saxon had feef fingers on each hand and we have five; and they wore boats on their fate and we wear boots on our feet. The Great Vowel Shift is still continuing today in regional dialicts; many speakers are now trying to move the topmost articulation points farther up, producing new diphthongs.
Why Was It?
There are theories for why the Great Vowel Shift has occured, but none are likely ever be testable without a time machine. Two models of the pattern of vowel change are the "pull theory" in which the upper vowels moved first and "pulled" the lower ones along, and the "push theory" in which the lower vowels moved forward and up, pushing the others ahead. Neither theory gives us an answer to why the Shift happened, and the actual shifting was so complicated by regional variation
that it will be difficult to ever sort out more than a general pattern of shifting. The regional variation of the shift has lead to a multitude of vowel pronunciations which are neither standard English nor standard Continental such as this anecdote:
Boy in NE England is sitting by a river, crying. Passer by asks what's up.
Boy says "Me mate fell in the water".
"Oh - that's terrible, how did it happen?".
"Fell right out of my sandwich, into the water!"
or the Cockney woman who, when trying to buy a cut loaf of bread was asked by the puzzled baker "is it a bread especially for cats?" Both of these examples are vowels that have shifted beyond the strict definition of the Great Vowel Shift. This is a demonstration that the English language is still evolving in wonderful (and confusing) ways.
In addition, the reconstruction of the sounds is based on texts, which are rarely a perfect means of recording sound. The printing press further complicated this problem, as it tended to fix spelling in the fifteenth and sixteenth century, before the sounds of speech had finished shifting (if they ever did finish). Today, we speak with twenty-first century pronunciation, but we write our words in a fifteenth century form.
Since the Great Vowel Shift did not occur in other languages or in some regional dialects of English (see, the Scottish House Cow, above), it is we Standard English speakers, not the foreigners, who have the wacky vowels.