how to pronounce 'a' in sentences?
I heard someone pronounce [eɪ]
but other sentences they pronounce [ə]
I really don't know how to pronounce 'a' now in a sentence

Question

Answer

I'd say 'eɪ' is used when you are speaking more slowly, or when you want to be more clear. 'ə' is easier to say,  and I would say is more casual, and so it is used in fast conversation :)

However, really you can use both whenever you like, there is no real correct answer.

Answer

Both are correct. You can pronounce 'a' either way but I think 'eɪ' is more casually.