A batsman gets stumped when he misses a ball and the wicket keeper catches the ball and knocks down the bails on the wicket with the ball. This can only occur if the batsman is over the bowling crease and has no part of his body or bad touching the ground behind the bowling crease.

Notice, it is not enough for the batsman to have his bat or part of the body on the bowling crease, it must be behind the bowling crease.

A batsman can not be called out at no ball, but with a wide ball.