Pronunciation of "a"

Solution 1:

If you mean ‘a’ the indefinite article, Ebenin, you’re quite right. In normal speech it is pronounced as a schwa, /ə/. Only when it is emphasised for some reason is it pronounced as /eɪ/. Explaining the reason fully would take more time and space than is available here, assuming any of us is qualified to do so.

Solution 2:

The letter a has several different pronunciations, which can make it difficult for learners if their language only has one, like for example, Spanish:

car -/ɑː/, cat - /æ /, what - /ɒ/, play -/eɪ/, and of course the schwa - /ə/ as in a book

I don't think there's much relationship between how letters are said in the alphabet, which is I think what you mean by a when it is single, and their use in language. But perhaps you mean a when it is the indirect article?

There is also the question of weak or strong stress: normal stress - "a book" (/ə/), but strong stress - "I said a book, not two books!" (/eɪ/).

And whether it is a stressed or unstressed syllable in a word, eg: marginal - first a stressed - (/ɑː/), the second unstressed - (/ə/)

Mastering English pronunciation is just one of those little joys of learning this specific language!

Solution 3:

Letters have "names". When we say each letter separately, for example when we want to spell a word, we use the "name" of each letter, like /eɪ/ for a, /bɪ:/ for b, /sɪ:/ for c, etc. But when we use the letter a in a sentence as a word, i.e. as the indefinite article, its pronunciation is /ə/ since the "name" isn't used any more. When we want to emphasize the article, then we pronounce it /eɪ/. The reason why a word (or a separate letter used as a single word) has a certain way of pronouncing it is not something I have come to understand.