Originally Posted by bob jenkins

If the foul is on the defense, and BEFORE the try has started (some use the word "gather'), then the ball is dead and the try does not count.

If the foul is on the defense and AFTER the try has started, then the offense is allowed to complete normal motions associated with the try. It doesn't matter that the foul was against a player other than the shooter.

If the foul is on the offense and BEFORE the ball has been RELEASED, then the ball is dead and the try does not count.

If the foul is on the offense and AFTER the ball has been RELEASED, then the try counts, unless this was a PC foul.