Best Answer - Chosen by Voting
Had I done that - Hadde jeg gjort det.. Had I won = Hadde jeg vunnet. Had I not eaten that = Hadde jeg ikke spist det....
If I did that - "Hvis jeg gjorde det.." or "Om jeg hadde gjort det".
"If I did that, would you be happy?" = "Hvis jeg gjorde det, ville du blitt glad?"