Ain’t is a short spoken form that can stand for am not, is not, are not, has not, or have not. I ain’t ready. She ain’t here. We ain’t got time. He ain’t done it yet. Many teachers, exams, and style books treat it as wrong, and you should not write it in school essays or formal letters. Though actually, millions of people say it every day — in parts of the United States, in some British dialects, in songs, and in films — and they are not “failing English”; they are using a real, old spoken form. It is first found in the early 1700s meaning am not. Later it spread, especially in London Cockney speech, as a general “not” contraction; writers such as Dickens used it to show that speech, which helped push it out of what schools call correct English. The sound also shifted, a bit like some dialects’ change in can’t. In careful English you want I’m not, isn’t, aren’t, hasn’t, haven’t. The polite question is aren’t I?, even though that looks as if it should be “are not I” — that spelling grew from a different pronunciation of the old am-not form. Use ain’t only when you are copying speech on purpose.