How to stop a dog from biting people?

When a dog bites, whether lightly or hard, stop it at once and act in pain so it knows this is wrong. Also train it and build authority, rewarding obedience with treats.

For a 3-4 month old in the teething phase that bites, give chew sticks, and when interacting, put a little bitter (e.g., bitter melon juice) non-toxic liquid on your hands so it learns human taste is unpleasant.