A serve into the wrong box
The serve must cross the net diagonally and bounce in the receiver's box. A serve into the box straight ahead is a fault, even if the receiver plays it back.
Diagonal every time
Every serve crosses the court diagonally into the box opposite you. The first point of each game is served from the right half of your court, and after each point you switch halves, so you serve into each box in turn.
Straight ahead is a fault
A serve that bounces in the box straight across the net has bounced outside the receiver's box, so it is a fault. So is a serve that lands beyond the service line.
The lines are good
A serve that lands on any line of the correct box, the centre line and the service line included, is good.
Playing it back changes nothing
The ball is only in play after a valid serve. If the receiver returns a fault out of habit, it is still a fault, and the server takes their next serve.
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Is the centre line in on a padel serve?
Yes. Any line of the receiver's service box counts as in, the centre line included.
Does a fault serve count if the receiver returns it?
No. The ball is only in play after a valid serve, so returning a fault does not make it good.