from The Origins and Development of the English Language by Thomas Pyles Ain't means am/are/is not. It is of late 17 th century origin . It was frequently used by everyone including highly educated people such as the editor of the OED, Frederick James Furnivell. It is correct to use it only in place of : (they/we) aren't, ( I'm) not, (It/he/she ) isn't So :" Ain't you ?= (aren't you) I ain't going. I ain't certain. " are all correct. --- But for some reason people now perceive it as uneducated , so I would avoid using.

The short answer is: 'Don't use it.' As for the long answer, 'ain't' is a contraction that some people use in place of am not, is not, are not, has not, and have not. It is not grammatically correct English. Many native speakers use it and some people use it rather a lot in informal speech. But to labour the point once more, it is not grammatically correct. It gives the impression that the person is lazy when they speak or that they are poorly educated. Native speakers will make that judgement about fellow native speakers who use this word. It is for this reason that I am highly reluctant to recommend to non-native speakers that it is an appropriate word to use. That may seem like a bit of a double standard, that native speakers can use it and non-native speakers can't. But for a learner it can be very hard to judge the correct context, so with this in mind my advice would be not to use it until your English is of a high level. And even then it would be preferable not to use it. If you insist on using it, definitely never use it in formal settings, and never use it in writing unless you want to specifically imitate colloquial speech.

As my teachers used to say "ain't ain't a word." You would use it in place of "is not," but it is considered very bad English. People who say "ain't" are generally considered to be uneducated.