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This is the level that says "White to move", meaning black has just moved. Pbafvqre jung oynpx cvrprf pbhyq unir zbirq, naq znxr gung vzcbffvoyr.

The knight has just checked the black king, so it must have just moved. This means black's queen was checking white just before. So black must have just made a move that put the queen in that corner square. Make it impossible for the queen to have moved there.

Man, Level 12 is making me unreasonably upset. In a vacuum, three pawns of either color can be in literally any configuration between the 2nd and 7th ranks, so something about the queen-king setup on the side matters...

But how can it matter? Both queens amd kings can get to those positions regardless of how the pawns are set, and vice versa.

The white knight can check black's king in a certain number of moves, but there are places black's knight cannot be when that check is delivered.

If you're stuck at level 7, the solution you've missed is probably double-checking the king.

Level 8 is about White to Move, ergo Black just moved — but there's only one black piece that could have just moved, make that case impossible.

Level 9, there's a certain place black's knight can't be while black is also in check.