Three words that get muddled in every beginner’s first month, and two questions that sort them out. Ask whether the king of the player to move is attacked. Then ask whether that player has a single legal move anywhere on the board. The four combinations cover everything that can happen in chess.
Check: attacked, but there is a way out
g The king is under attack and something can be done about it. Play does not stop, but Black’s choice is narrow: nothing on the board matters this move except the queen’s diagonal. There are always three kinds of answer, and here only one of them is available.
Checkmate: attacked, and no way out
Same king, same corner of the board, one move later, and the game is finished. Black is attacked, cannot take a queen guarded by the king on e6, cannot block a piece he is touching, and has no square left. Mate is written with a # and it ends the game the instant it appears, so nobody ever captures the king.
Stalemate: not attacked, and no move at all
f This one is the cruel one, because Black is a pawn from queening and gets half a point instead. White is not in check, so the position is not mate. White has no legal move, so play cannot continue. Draw.
What saves White is whose turn it is. Hand the same position to Black and he wins: 1…Kg3 leaves White the single legal move 2.Ke2, and after 2…Kg2 the pawn promotes next move. A position where the player to move would give anything to pass is called zugzwang, and this three-piece one decides more endings than any other.
If you want the three ideas drilled rather than read, mate in one trains the second one and punishes the third: half the failed attempts in those puzzles are moves that would take the enemy king’s last square away without giving check.