One problem is that English letters have names, like /ai/ for a,
but often several sounds (and, any, able, father).
The main sound for a is as in 'at, and, bat, hand, pat, patter ...),
for e as in 'egg, bed, leg, leggy...' , for i as in 'in, it, sit, bitter...',
o - 'on, hot, spot, spotty...., u - up, cup, nut, nutty... .
In those words they have what is often called a short sound.
They mostly have a short sound in words ending with a single consonant (at, bed, bit, hot, cup), several consonants (bend, cost) or before a doubled consonant (batting, cupping) or.
In such words a, e, i, o and u are also called 'closed' by some.
When a, e, i, o and u are not followed by a consonant (he, go) or by single consonant and another vowel they are 'open' and mostly have a long sound (make, zero, bike, solo, mule). - Over a thousand common words spell the /ai/, /ee/, /igh/, /oa/ and 'ue' sounds by that principle.
But there are several hundred exceptions as well:
100+ words have -ve endings even when the vowel is short (have, give, defensive).
-ate endings in longer words can be short or long (delicate, inflate).
Quite a few others also don't obey that rule (apron, very, kind, only, study), but the majority of words spell short and long a, e, i, o and u by the closed/short or open/long rule.