Re: Idiomatic expressions
Welcome to the forum. Aletz-black.
Ain't is not really an idiomatic expression.
It's rather the way some people produce am not/are not/is not/have not/has not in informal conversation. It is usually considered sub-standard, 'uneducated', but there are a few expressions in which even educated speakers may use it. These include: "If it ain't (= isn't) broke, don't fix it" and "You ain't (= haven't) seen nothing yet". You could call those two expressions idiomatic, in that they defy the 'rules' of grammar, but are commonly used, and accepted, though not in academic wrting.
I recommend that you do not use ain't.
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