The letters a, i, o, u (and sometimes e too) have two common short and long sounds, as in:
flat, flatten – inflate; hid, hidden – hide; dot, dotty – dote; tub, tubby – tube; (hem, hemmed – theme).
I have explained this short / long (or closed / open) vowel spelling system in http://improvingenglishspelling.blogspot.co.uk/2010/06/long-and-short-vowels.html like this:
When a, e, i, o and u are followed by just one consonant, or several consonants and a vowel, they are 'closed' and are supposed to have a short sound, as in:
am, ample, ten, tender, pin, pinked, drop, droplet, bun, bunting.
If a single consonant after a, e, i, o and u is followed by a vowel, they are supposed to be ‘open’ and long, as in:
hale, halo; peter, period; fine, final; sole, solo; tube, tubular'.
If a stressed vowel before a single consonant and another vowel is to stay short, it is supposed to be followed by a doubled consonant:
attitude, petty, pinnacle, dotty, bunny.
Hence: cut + er = cutter, prefer + ed = preferred, enter + ed = entered, cute + er = cuter.
Thousands of English words conform to this system. Unfortunately, there are also hundreds words which break the ‘closed /short' and 'open / long’ vowel spelling method in one or more of five different ways. ...
My blog http://englishspellingproblems.blogspot.co.uk/2013/11/irregular-spellings-in-4217-common-words_7864.html shows all common words which do so.
The only English vowel spelling that has a completely regular pronunciation is ee, as in 'keep sleep deep'. All others have some exceptions - http://englishspellingproblems.blogspot.com/2009/12/reading-problems.htm.
Beyond a very basic level, pupils have to learn to read and write English words one by one.