Hi, Were I can found information about Idiomatic expressions?, I have many doubts with this. For example: Ain't
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.
Aletz-black, here are two ways of finding further help on this topic from the forum:
1. Look below and you will find Similar Threads on the subject, as this question has been asked before.
2. Near the top of the page you'll see Reference. Click on that and then click on 3,683 English Idioms.