Seems like the others already explained it's slang, so here's how to use it: A: "Wow, this car is great!" B: "You ain't seen nothin' yet, my car is way better!" You could also replace it with: "That's nothing compared to my car!" "You think this is great? Wait until you see my car!"

"seen" is correct, but "ain't" is a slang term that you would not use if you are writing. If you are going to say it, take the 'g' from 'nothing'. You ain't seen nothin'

"You ain't seen nothing". I wouldn't use it, it's more of a slang term.