A forward pass, legal or illegal, is incomplete and the ball becomes dead when the pass touches the ground or goes out of bounds. It is also incomplete when a player in is the air possesses the pass and alights so that his first contact with the ground or with anything other than a player or game official is on or outside a boundary. When an incompletion occurs the down counts unless the pass is after a change of possession. If the pass is legal, the passing team next snaps the ball at the previous spot, unless lost after fourth down.
Tag: illegal
Rule 9, Section 4, Article 4
Roughing the passer. Defensive players must make a definite effort to avoid charging into a passer, who has thrown the ball from in or behind the neutral zone, after it is clear the ball has been thrown. No defensive player shall commit any illegal personal contact foul listed in 9-4-3 against the passer.
Rule 10, Section 2, Article 6
The following fouls by A include loss of the right to replay a down:
a. Illegally handing the ball forward.
b. Illegal forward pass.
c. Illegal touching of forward pass by an ineligible.
The Zebra Files 