Here is an important twist. Black’s most advanced knight is on a dark square. So are White’s queen, king, and rook, with the latter two pieces subject to a fork at c2. But c2 appears to be protected by White’s queen. The queen is not constrained by a pin—yet. But examine the fork by actually playing it in your mind’s eye, imagining the knight on c2 and not on b4. When you so imagine a move or exchange, pay attention to what lines are opened and closed by it and what consequences may follow—especially new pins and new possible checks. In this case, once the knight moves the White queen is pinned by Black’s queen. So play goes 1. …Nc2+; 2. Kf1, QxQ (without this intermediate step, all is lost; do you see why?); 3. NxQ, NxR. This time the lesson is that you do not just ask whether the troublesome piece currently is pinned; you ask, too, whether it would be pinned if you made the forking move.