“Ain’t” has a long and vital history as a substitute for “isn’t,” “aren’t” and so on. It was originally formed from a contraction of “am not” and is still commonly used in that sense. Even though it has been universally condemned as the classic “mistake” in English, everyone uses it occasionally as part of a joking phrase or to convey a down-to-earth quality. But if you always use it instead of the more “proper” contractions you’re sure to be branded as uneducated.