"Ain't" is an informal way of saying "is not/are not/am not"... it is considered to be bad English, and should not be used.

"ain't" isn't an actual word- it's slang for 'isn't' , 'am not', or 'are not'. There's no time when you'd use 'ain't' that you couldn't use one of those'; it means the same thing. If you decide that you wan't to use "ain't" make sure it's in an informal setting, and avoid using it in writing. --- hope that helps :)

"Ain't" is colloquial, informal English. DON'T TRY TO USE IT for two reasons. First, it's not considered good standard English. Second, its use is regional and somewhat rustic. If you did not come by it naturally--learning it while you were growing up--people will think you are a phony, pretending to be something you are not. It IS an actual word. In 1961 Merriam-Webster included it in "Webster's Third New International Dictionary." They warned that it was "disapproved by many and more common in less educated speech," but they included it. This bothered a lot of people. For decades, English teachers had been saying "'Ain't' ain't in the dictionary," and now it was. In my high school an English teacher put up a whole display of reviews and newspaper clippings, on a bulletin board in the hallway, by people arguing about it. Although you SHOULD NOT use it, the English language actually NEEDS this word The reason is that "ain't" is a contraction for "am not," and there isn't any other! "Ain't I glad to see you?" is uneducated, nonstandard English. Educated people say "Aren't I glad to see you?" But if you think about it, IT IS TERRIBLY WRONG, because "Are I not glad to see you?" is obviously bad grammar. "Amn't I glad to see you?" seems like the right thing, but there simply is no such word as "amn't," not standard English, not colloquial English, not slang. "Am I not glad to see you?" is perfectly correct--and yet wrong, because it sounds stilted, pompous, and old-fashioned.