how to pronouce "a" before a noun in American accent?

[Lock]

for instance:

a bag

a special thiing

a beautiful woman

we all know that in british accent, people say [ə]

and i find that in american movies ,people often say [ei] ~~but not everytime

so i wanna figure out when we pronouce [ei] befoer a noun~

thank u in advance!

[Beeesneees]

British English pronunciation is not as clear-cut as you would like to believe, Lock.

Please don't write 'wanna' and 'u'.
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Cheers m' dears!

[HirokoJoya]

Yes, I have very hard time with this too. Still not sure how to remember.