As far as I understand, "ain't" can mean either "isn't" (ain't no sunshine) or "hasn't" (you ain't seen nothing yet).
Are there any rules when "ain't" is used? Does it have a different meaning than "isn't" or "hasn't"?
Edit: Maybe asking for rules was not the best idea. A better question would have been: when do you use "ain't" instead of "isn't"/"hasn't"?