Quote Originally Posted by andrewsco
Ты не знаешь, где мой футбол?
In this sentence, which I know understand to mean 'do you know where my football is?' why is there an 'не'? What exactly is it negating? Well, actually why is it?

Ты знаешь, где мой футбол? Could you not just put this? As in if you were to ask Do you speak russian you would say:
Nooooooooooooo. don't let anybody confuse you!

DO NOT SAY: Ты знаешь, где мой футбол?
DO SAY: Ты знаешь, где мой мяч? (or, футбольный мяч)

футбол means football (the sport not the object)