Spanish grammar question.?

Normally I would write the sentence "do you know why?" in Spanish as "Tu sabes porque?". However I just started thinking that my translation leaves out the "do". i then started thinking that perhaps a more proper way of saying it would be "haces tu sabes porque?'. It seems like a lot of verbs in the "yo" form so it doesn't really seem right. However, when I typed it into Google Translates it gave a more grammatically correct English sentence. Which one is better? Should I leave out the "do" part in all my sentences?