THE GREAT VOWEL SHIFT

One reason why English spelling looks odd to people literate in other languages is that the pronunciation of long vowels has changed radically since the Medieval period, the "Great Vowel Shift", but the spelling has hardly changed. It is rather unclear to phonologists why these changes (to be given below) happened, but we can at least say with pretty high confidence what happened.

E.g. 1. Old English nama "name" had an open vowel, like "aah", but modern English "name" has a mid, front vowel, so the change was something like this (listen): http://www.ancientsounds.net/GVS/nAAm-to-name.wav

E.g. 2. Middle English feh "fee" had a mid, front vowel, but modern English "fee" has a close, front vowel, so the change was something like this (listen):
http://www.ancientsounds.net/GVS/fEE-to-fee.wav

E.g.3. Similarly, Middle English "deep" had a mid, front vowel, but in modern English "deep" has a close, front vowel, so the change was something like this (listen): http://www.ancientsounds.net/GVS/dEEp-to-diip.wav

E.g.4. In Middle English, "I" had a close, front vowel, but in mod. English it has lowered and become a diphthong, starting quite open but still close in its latter part. For many speakers it also begins quite back now:

http://www.ancientsounds.net/GVS/ii-to-ai.wav

So for the long, front vowels:

1) the open vowel [a:] became mid [e:] (later, [eɪ])

2) the mid vowel [e:] became close [i:]

3) the close vowel became open, diphthongized to [aɪ] (then [ɑɪ]).

I'm not claiming it happened in that order though; that is a tricky question.

The long, back vowels also changed:

1) open [ɑ:] became mid [o:] (later [oʊ] or [əʊ])

2) mid [o:] became close [u:]

3) close [u:] became open, diphthongized to [ɑʊ] then [aʊ] in some dialects.

It wasn't the same in all dialects, so e.g. American, Scots etc are different.

E.g. 5. Old English gāst, "ghost", had an open vowel like "aah" (it survives today in some pronunciations of "ghastly"), but now it's a mid, back vowel -- pure [o:] in some dialects but diphthongized and sometimes centralized in others:

http://www.ancientsounds.net/GVS/gAAst-to-ghost.wav

E.g. 6. Old English gōs, "goose", had a mid vowel, but now it's a close, back vowel [u:]:

http://www.ancientsounds.net/GVS/goos-to-guus.wav

(In this simulation the vowel of "gōs" is a bit diphthongized, which may not be accurate. Still ...)

E.g. 7. Old English hund, "hound", had a close vowel, but now it's an open, back diphthong:
http://www.ancientsounds.net/GVS/huund-to-hAUnd.wav

E.g. 8. Likewise Old English brū --> Modern "brow": 

http://www.phon.ox.ac.uk/jcoleman/eastern-origins/GVS/bru-to-brow.wav

This may seem complicated, but it's a quite oversimplified version of the story! Specialists will know of further details, and considerable variation in different dialects. I just wanted to illustrate the overall picture of the Great Vowel Shift using audio.