One rule which all English-speaking children learn at school is what some primary teachers call the 'magic e'. If you add 'e' to the end of a word, it changes the sound from a short vowel to a long one. For example:
hat /hæt/
hate /heit/
nap /næp/
nape /neip/
A doubled consonant also indicates a short sound.
lack /læk/
lake /leik/
tapping /tæping/
taping /teiping/
I can't think of many cases where an 'a' is pronounced /e/, other than certain one-off irregular words such as 'said' /sed/ and the past form 'read' /red/, which you just have to learn.