Hit is used much more often. Basically anytime two objects come in contact with force. Ex: My arrow hit the target. The ball hit him on the head. I got angry at my brother so I hit him. Knock has two main uses that are more specific. 1: When you hit something in order to make noise. Ex: I was knocking on the door. (Usually to signal to the people inside that you are there.) 2: The phrase "knock over" which means to hit something so hard that you make it fall. Ex: I knocked over the lamp. I knocked over my glass of milk and made mess. Hope this helps!

To knock is used most frequently in the context of getting someone's attention such as "I knocked on the door before entering" or if you are clumsy such as "I knocked over my glass of water" . To hit on the other hand is more aggressive such as "I hit the baseball with the bat" or "I hit my car with my bike by accident"