A piece is "hanging" when it can be captured for free, nothing recaptures it, nothing defends it. Most games below the master level are decided by hanging pieces, not brilliant combinations, so this is the single highest-value skill to build first.
Before you calculate anything else on your move, scan the board for any piece, yours or your opponent's, that a piece could take for nothing. Do this on every move, every game, until it's automatic.
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a
White's rook on a1 has no defender. Black to move.
Which move wins material for free?
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a