I'm really confused about the pronunciation of the letter "a". Why is there a difference when it is used in a sentence and when "a" is single? When it is single, we read it like dwelling on it, like /eɪ/. On the contrary, in a sentence, it sounds interruptedly. What's the reason?
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.
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!
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.