3 Answers

Is it a full grown dog or a puppy? If it's a full grown dog and you can not train it to stop biting, then it needs to be put down. If it's a puppy then that's how they play. Each time it bites, pull your hand (or whatever he's biting) away and say no, then give it a toy to bite Do not yell but be assertive. Each time he bites, you must do this. The puppy will figure it out in time that biting a body part is a no, no but biting toys is OK. 

Do you have any idea why the dog is biting?

This is a common behavior of dogs which were taken from the litter too soon.  You see they must have "puppy play" with their litter mates.  This is play-fighting, and it has consequences, that is, if they hurt each other, they inflict slight pain, and they learn from this not to bite!  If you have ever watched a mother dog with pups, she too will scold them with biting to just hurt them to teach them.  They learn that this is unpleasant, and do not do it to others, including people.   Some trainers say to initiate puppy play, and then when they bite, to pinch their mouth and cause pain, and say no.  They then learn what they should have as pups.  I have not tried this, so be careful, or have a trainer do it for you.