pronunciation article A

HI, I posted this already, but didn't get any answer. I hope to be luckier this time.
When I have the article "a", for example A one-time thing, or A waffle, etc..., when do I pronounce A (as in pArt) and when EI (as in gay)? I hope that the question is clear.

thank you very very much.

[Buddhaheart]

Pronounce ‘a’ (the indefinite article) /eI/ as in ‘day’ or /F/ as in ‘at’ when stressed and /c/ (the schwa sound) as in ‘ago’ when unstressed.

I do not know when do you pronounce ‘a’ as that in ‘pArt’ /pYrt/. As an article, I guess you don’t.