The 'e' in leg has the same pronunciation as the 'e' in bed and hen
If you are pronouncing it as 'lig' then you have been misinformed.
E gives an 'i' sound is a small number of words one of them being the word English, another pretty.
Where you usually get the change of sound is when there is a following (consonant - vowel) combination such as mere or penal - in both cases the 'E' here is pronounced as EE (Say of CHEESE to smile)
E can also sound 'er' as in were and her - the e sounds like fern (which rhymes with turn)
and 'ea' as in there which rhymes with care and bear
The really tough part is that there is no easy rule to tell you which is the correct pronunciation - It is a matter of learning and remembering which is which.
E can be a bit of a last resort vowel - When the first dictionaries were being compiled and decisions were made on how to write a word which had been spoken for centuries - the letter E often seems to have been put in just because none of the other vowels seemed to fit.