My dog keeps biting?

I just bought a chow chow-shepard mix about 3 weeks ago and he keeps biting i know its normal for them to bit but he crosses the line like he doesnt play bit like his biting really hurts! I have lots of scars from him biting me and he just bit me today really hard and it left a really deep cut that was bleeding alot so please advice to hlp because this dog is getting on my nerves!