Theory

A good starting point to understand English pronunciation is to consider how the VOWEL sounds work.
The English alphabet has 5 vowels A, E, I, O and U, but these letters are used to generate over 15 different sounds.
This is because some of them are LONG sounds and some are SHORT sounds.
For example:
FOOD is a long ooo sound AS IN BOOO.
FOOT is a short u sound as in PUT.

The second point to consider is that each vowel can give at least 2 sounds, the sound of its NAME, and the sound it REPRESENTS.
For example:
A can give the sound of FACE which sounds like its name A.
A can also give its natural sound of A as in CAT.

The situation is further complicated because sometimes one vowel can give the sound associated with another vowel.
For example:
U gives the natural sound of U as in BUS.
but O can have the same sound as in SON.
O can have the natural sound of O as in DOG.
but A can have the same sound as in WALL.

It is therefore impossible to have fixed rules about how each vowel should sound in any given word.
The best method to use is to treat each word individually and to learn its correct pronunciation.

The letter Y sometimes also behaves like a vowel.
For example:
Y in the word BOY sounds like an E.
Y in the word WHY sounds like I.

Another complication is that some vowels are SILENT, which makes it difficult to predict their sound.
For example:
The E in FACE is silent.
The Y in DAY is silent.