What knights can't do

There are certain positions in a game where it seems a knight can do something, but it can't.  It would be quite useful to know and memorize those positions so during a game, you don't waste your time finding ways to attack with the knight.  

As you see, in all these diagrams, the knights are right next to the squares they should be on so that they can attack their targets in one move.  These postitions are all important to remember, so you can know that in those positions, there is no way to attack the target in one move.  That way, it saves you time and brain power for the rest of the game.