I see so many times in a motion offense against a man to man where an offensive player will step out on a screen and pick off a defender who may be trailing the offensive player. The offensive player is never set. It seems like the foul is never called. What is the rule? I would think the screener has to give so much room and be set.

You're correct for the most part however, there are some exceptions to keep in mind. First of all, yes, the screener must always be set. Whether or not he is required to give any space between his position and the player he is trying to screen depends on whether or not he is within the visual field of the defensive player. If he sets his screen outside the defensive players field of vision then he must give him up to 2 steps to avoid the screen. If he sets up in front of the player where he is seen by the defensive player then, as long as he is there first and is set, the defensive player has the burden to go around the screen.

The first reply is incorrect.

For setting a screen in the visual field of a stationary opponent, no time or distance is required.

For setting a screen outside the visual field of a stationary opponent, a distance of 1 normal step is required.

For setting a screen on a moving opponent, time and distance are required depending on the speed of the opponent, but never more then two steps.

I think Jerry's question is specifically referring to screening a moving opponent, with a player stepping in front of the defense at the last moment to cut him off. This does happen fairly frequently, and unless the defender is running backwards it almost always occurs within his visual field. If it happens that quickly--that is, just as soon as the screener jumps in and is (or is not) set, a collision occurs--then it is an illegal screen that should be called, because the defense had no chance of avoiding contact. He must be allowed that step or two to react. Of course, if the defender has REALLY quick reflexes and does manage to mostly stop or somehow avoid major contact without displacing the screener, then we can pass on a foul.