why does my dog bite my cat?

Well, I have a dog, and I have always guessed that the dog sees a new animal, and wants to play with it, the cat doesn't like that, so it scratches the dog or whatever, and then the dog just gets mad at it. And to stop it: next time you see your dog do this, discipline it. The way I always discipline it is by holding(and applying a bit of pressure) it against the ground on its side. And in the dog's mind, he sees you as a sort of "pack leader".

SUPERGLUE the cat to the dogs back for a week. (((((( NO ! ! ! DON'T DO THAT ! ! ))))))

Your dog doesn't like the cat OR it is trying to get the cat to play. Your total lack of detail doesn't help us much here.

If this is play the cat will remind the dog when he's getting too rough.

If the dog hates the cat you have to train it to accept it. I have a dog that used to kill and eat cats. He's sleeping in the living room right now with my two cats and I can assure you that he's not going to even growl at them.

Dogs and cats are natural enemies. You can't really stop it.