Forks

A fork is one piece attacking two or more enemy pieces in the same move, so the defender can only rescue one of them. Knights are the classic forking piece -- their L-shaped move lets them attack squares that look completely unrelated to each other, which is exactly what makes a knight fork so hard to see coming -- but any piece can fork: a queen raking two rooks along a rank and a file at once, a pawn attacking two pieces diagonally, even a king forking two undefended pawns in an endgame.

The most valuable forks attack the enemy king together with something else, since a check must be answered immediately -- the opponent has no time to save the second piece. "Family forks" that hit the king, queen, and a rook all at once are the dream version of this pattern.

When you're scanning a position for forks, look at every one of your pieces and ask: if I moved this piece to square X, what would it attack from there? Knights especially reward this kind of active, one-move-ahead scanning, since their attack squares aren't visually obvious the way a bishop's diagonal or a rook's file is.