Question: can the ball hit the net on a serve in tennis

Answer: Yes, A let occurs when a legally delivered ball lands in the cross-court service box having touched the net cord (but not the net post or any other object). Play stops immediately, and the server is allowed another attempt. The umpire (or opposing player when there is no umpire) will usually say ``let'' or ``net'' to signal to the server a let has occurred. Lets occur more often on first serves, as the server is more cautious on the success of a second serve.