White’s knight is on a dark square. Black’s queen and knight are on dark squares (and also the king, but at the moment it’s too far away to fork). So you need to see that White has a potential fork at e4 but that one of the targets is Black’s knight, which won't do. Ask whether White can take the knight with something else and thus cause it to be exchanged for a suitable target. He can, by playing QxN+; if Black responds KxQ, White has Ne4+, forking king and queen. He ends up gaining a knight. Throwing away your queen to take an enemy knight looks counterintuitive, but a temporary sacrifice of this kind is a common opening thrust in a tactical sequence.