These problems are HARD! But first, what in the world is an "impossiproblem?" In a nutshell, it's a puzzle where I give you a diagram and in return you have to tell me whether:
a.) This position CAN be reached in a chess game
b.) This position CAN'T be reached in a chess game
Good To Know: Sometimes the best way to solve one of these problems is to backtrack and see, "what could the previous move have been"? This works the best on double check problems, as I'll be showing in an example.
ALSO: ALL SPECIAL MOVES NEED TO BE CONSIDERED
THIS INCLUDES STALEMATE, CHECK, DOUBLE CHECKS, EN PASSANT, CASTLING ON EITHER SIDE, AND PROMOTION.
Here is an example, that is very easy, and incorporates everything I have said so far:
(Answer) Looking at the board, you see two queens, each one checking the king, in the middle of the board. This is IMPOSSIBLE because the only way two queens can be doing this is when a pawn promotes to a queen, and it reveals a discovered attack by another queen. If you try to backtrack, you will find there is no move black could have played before to reach this position.