Lesson
White to move. Main line: 1. g7#
g7# checks the king on h8 because a white pawn attacks one step diagonally onto h8. The pawn is protected by the king on f6, so Black cannot capture it even if the king could reach g7.
The bishop on e6 controls g8 and the bishop on c2 controls h7. Those two long-range pieces remove the only flight squares, leaving the humble pawn to supply the check that ends the game.
Pawn attacks are easy to misread because they differ from pawn movement. In mating calculations, mark the two diagonal attack squares of every advanced pawn before examining the more visually powerful pieces.