Here are the rewritten sentences:

37. David is tired; therefore, he can't score any goal.
→ Because David is tired, he can't score any goal.

38. Buy her a puppy or she will cry all day.
→ If you don't buy her a puppy, she will cry all day.

39. My nephew broke the plate. I received the plate as a present.
→ My nephew broke the plate that I received as a present.

40. I don't play the piano as well as he does.
→ He plays the piano better than I do.