Checks, then captures, then threats
A position has too many moves to look at one at a time. So look at them in an order. Every move that gives check, first. Then every move that takes something. Then the quiet moves that threaten something next turn.
The final mating move gives check. For mate in one, examine every check and all possible replies. A mate in two can start with a quiet move that prepares the final check, as this example shows.
Before any of that, count the king's escape squares. A square its own piece is standing on is not an escape. A square one of your pieces covers is not an escape. What is left is the list a check has to close.
Positions from the Lichess puzzle database, released under CC0.
A quiet move that takes the last square away
The knight can go to g6 and give check. It looks like mate, because g8 is covered by the pawn on h7.
Count again. The knight has left f8, so nothing defends the pawn on h7 now. The king has g7, which is empty, and h7, which is the pawn itself. Either answer ends the check.
So the first move is not a check at all. The White king steps to h6. That one move covers g7 and defends h7. Now the king on h8 has nothing, and whatever Black plays, the knight goes to g6 and that is mate.
Why is the check not mate?
Go back to the first board. The knight is on f8 and the White king is still on g5.
The knight goes to g6 and gives check.
Try it
The knight goes to g6 and gives check. Why is it not mate?
- The king can go to g7, or take the pawn on h7
- The king can go to g8
Show the answer
The king can go to g7, or take the pawn on h7
Correct. This knight check leaves both g7 and h7 available. The White king to h6 closes them together: it covers g7 and it defends the pawn on h7. Then the same knight check is mate.
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