To copy my answer from [ http://www.italki.com/question/206349 ] When writing an article, an assignment, etc. words like "ain't", "gonna" or even "isn't" should not be used. In daily speech, contradictions like "wasn't", "haven't", etc. are fine. Words like "gonna" and "wanna" are best avoided when typing (or else you may seem like a youngster) but are naturally spoken at times. However, "ain't" should be avoided at all costs. It can mean "haven't", "hasn't" "aren't" or "isn't" (maybe I missed some). It's very lazy to use this and implies you don't know which contraction to use. -------------------------------------------------- So when should you use it? Never, unless you are trying to sound less intelligent. How often is it used? It depends with whom you speak. It's used by the kind of people who don't know the difference between "you're" and "your".

The only time you should use substandard English is when reading from published text.

I will disagree with everyone else here and say that "ain't" can be appropriately used without automatically making a person sound uneducated. Typically, folk music and country music (which I personally don't like, but whatever...) accept the use of "ain't." Also, it can be acceptable in poetry if the tone of the poem allows for it. It obviously would not be used in a sonnet, but it can be used in more poetry with more casual language.

I would say never because it makes you sound uneducated. It is even worse when a non-native English speaker says it.