The grammatically correct form of "ain't" depends on the context in which it is used, as "ain't" can serve as a contraction for several different phrases. It can stand for "is not," "are not," "have not," or "has not."
For example:
- "I ain't going" can be grammatically expanded to "I am not going."
- "They ain't here" can be expanded to "They are not here."
- "He ain't finished" can be expanded to "He has not finished."
In each case, the grammatically correct version will change based on what "ain't" is intended to convey in the dialogue.