how do u check mate with a knight and bishop

It's hard to explain. You can only force checkmate in a corner the same colour that the bishop is on but you usually have to start by forcing your opponent into a corner of the other colour.

There is a video here that gives a fairly good explanation.

google chess tablebases and it will show you ALL the moves no matter what your opponent does , for K+B+N Vs K

Asked and answered as recently as 3 days ago.

http://www.chess.com/forum/view/endgames/forced-checkmate-with-bishop-and-knight-vs-king