Tennis - Service sequence
Service, lets, and foot faults without the muddle.
The serve starts every point, but the ruling is not just "in or out". Officials also judge whether the server used the correct order, whether the ball clipped the net and still landed legally, whether a foot touched the wrong place before contact, and whether the point dies immediately or must be replayed.
Quick ruling: first ask whether the server was entitled to serve and stood legally. Then ask whether the ball landed in the correct box. Only after that do you decide whether the point continues, is replayed as a let, or is lost by fault.