Weak Squares and Weak Pawns
What Makes a Pawn Weak
A pawn is weak when it cannot be defended by another pawn and must instead be guarded by your pieces. Pawns are best at defending each other in chains; a pawn cut off from that support becomes a long-term target your opponent can pile up against.
The Isolated Pawn
An isolated pawn has no friendly pawns on either of the files next to it. Because no pawn can ever defend it, it must be protected by pieces, and the square directly in front of it makes a perfect outpost for the enemy. Note that an isolated pawn can still be strong in the middlegame if it grants you active, well-placed pieces.
Doubled Pawns
Doubled pawns are two of your own pawns stacked on the same file, one in front of the other. They cannot defend each other, the rear pawn is often hard to advance, and together they control fewer squares than two pawns side by side would. They do, however, open a half-open file for your rook as compensation.
The Backward Pawn
A backward pawn has fallen behind the pawns next to it and can no longer be safely pushed forward, because an enemy pawn or piece guards the square ahead of it. It typically sits on a half-open file, where enemy rooks can stack up and hammer it, while the square in front becomes another enemy outpost.
Targeting and Avoiding Weaknesses
To attack a weak pawn, fix it in place so it cannot advance, then attack it with more pieces than can defend it — and occupy the weak square in front of it with a knight. To avoid weaknesses of your own, keep your pawns connected and think twice before making a pawn move you can never take back.