Jorden, "Ain't" has only been accepted in recent years as useable grammar. It is used as a contraction for "are not", "am not" and "is not" Old-timers like me still do not accept it as proper grammar. It is a slang term that has been accepted by common usage. When I was young, all students were taught that "ain't" was not a word. To use it as in your first example would still be considered improper grammar, because it uses a double negative. (Most of the people who use it like that would say, "They ain't got no future".) Your second example, though still denied as proper grammar by my generation, would be the correct usage.

Thank you guys. The information you gave is really useful. In fact I'm not used to use ain't in my grammer, but what made me wondering that I hear it a lot in tv, newspapers, magazines.....etc. And I thought it might be stronger in expression. However, esl Larry, about what you said that old-timers don't use ain't, I listened to many classic songs for famous singers and they all use it.

ain't = isn't; aren't; doesn't; don't; haven't; hasn't He ain't got no future. This is common slang and you hear it a lot in pop music. It is grammatically incorrect, as there are two negatives. But this kind of slang is used a lot.

They are both slang, and therefore both incorrect and should not be used. Instead, you should say, "They don't have a future" and "I'm not dumb." However, if we're going to choose which one is more wrong it would be, "they ain't have no future" because 'ain't' is negative as well as 'no' ... so they cancel each other out and really that sentence says "they have a future." This is called "double negatives." Please don't use 'ain't' when talking English though. :)