On your serve, did the ball touch the net, the center strap or top tape before hitting the opponent? If so, the ball is considered a let. Why? Because for the ball to be considered out, it has to hit some part of the court after it touches the net, center strap or top tape or net cord.
If it did not touch the net or any part of the fixed court (posts, singles sticks, etc), did the ball cross the net and hit the ground in the correct service box BEFORE it hit your opponent? If the serve was good and the ball hits the opponent receiver or opponent partner, the point is yours.
If the serve did not land in the correct service box, THEN hit your opponent receiver or opponent partner, then the ball was considered out of play (or dead) before it hit the player. If this is the case, you can start your second serve. If that was your second serve, you lose the point and begin your next serve.
IF the ball hit your opponent BEFORE it hit the ground or net, the point is yours. After the ball crosses the net, it needs to hit hit the ground before it is determined to be “out” or “in”. If the opposing players prevent the ball from hitting the ground first, they lose the point and you win the point. It is not so much as the opposing players PREVENT the ball from hitting the ground first as much as they just could not get out of the way of the ball fast enough.
It is odd but if you hit your opponent with the ball, you win the point. Maybe that is why so many players try to hit their opponents.
See you on the courts!