Puppies are babies. They have not learned the rules yet; in fact, a puppy does not learn how to properly interact until about 16 weeks of age. Puppies are teething, and it's instinct for them to test boundaries. If they stay with their litter mates longer they tend to learn bite inhibition naturally….however:
There are a couple different ways you can teach your puppy "bite inhibition" - the most common way is to 'yelp' when the puppy bites too hard, and to immediately stop playing or interacting with it. This is what mom, litter mates, and other dogs do when a puppy gets too rough.
I like to stand up and turn my body away; dogs [and puppies] are social animals so shutting off any interaction makes it very clear to them very quickly what they should NOT do. If you keep playing with them they will not get the hint. You will have to do this quite a bit, repetition is key to them learning.
There are other more 'creative' methods I've heard of being used by people on these forums as well…. maybe they'll mention them here.
Another method is to switch out your hand for a toy. Yelping after being bitten too hard is pretty key, but then switch your hand for something more appropriate to chew on.
Another method would be to let the puppy chew on your hand, but then yelp and immediately stop playing with them if they chomp down.
Puppies need lots of things to chew on while they are going through this stage.
My dog has a very soft mouth (softer than any retriever we've had even!) I used the first method mentioned above [ but he also stayed with litter mates until 9ish weeks] most people swear by that method.