QB throws ball and lands behind line of scrimmage without being caught by either team an hits the ground
The ball landing beyond the line of scrimmage is irrelevant, for the most part, in this question. The thing that is relevant is whether the pass went forwards from the quarterback's location, or backwards. The quarterback must be behind the line of scrimmage, of course.
A forward pass that hits the ground is an incomplete pass, which ends the play and no gain or loss is recorded - the next down will be at the same location as the prior down.
A pass that hits the ground either directly sideways from the quarterback, or behind him, is treated as a fumble, and is able to be recovered by any player on either team, at which point the ball can be advanced; once downed, the next down will begin from that location.
There is one exception: if the quarterback intentionally throws the ball into the ground, not near a receiver, it might be considered intentional grounding. A quarterback has two exceptions that apply to the intentional grounding rule:
- If the quarterback is outside of the tackle box (meaning, to the right of the right tackle, or the left of the left tackle, in the position they initially set up in), and throws the ball beyond the line of scrimmage, it is not intentional grounding.
- A quarterback is permitted to snap the ball and immediately throw it directly down into the ground, in order to stop the game clock (in a situation with little time left).
If the quarterback is not stopping the clock, and throws the ball intentionally awry (not near a receiver), and it lands behind the line of scrimmage, then it is intentional grounding, and treated as if the quarterback had been downed at the location he threw the ball.