From what I know, "ain't" works as a negation in any tense or form. However, it doesn't take the form of the (first/second/third) person or the tense and so the verb following it does. What I mean is:
- He didn't help me.
- He ain't helped me.
- He ain't help me.
In (1) "did" already takes the form of the correct tense and person (in this case the same for all persons) and the verb following it ("help") is infinitive.
In (2) "ain't" doesn't take the form of the person or tense and so the verb ("helped") does.
The alternative (3) would be for the verb to stay infinitive, but as far as I know that is incorrect. Also, in this case there's no way to determine the tense someone is using (other than from the context).
Now, am I correct or am I wrong? Or maybe in different dialects (or slang varieties) "ain't" behaves differently?
Edit: To clarify—I know what "ain't" means, I'm asking about how to use it correctly in a sentence.