What are weak pawns?
Basically all pawns that can't be defended by another pawn are weak, but some pawns are weaker than others.
The pawns on half open files are the easiest targets.
In this position, the pawns b6, c4, e5 and f7 are the easy targets. The a4 and g3-pawns are weak as well, but they're a little less accessible.
The tripled pawns are very weak, as is the d7-pawn. The black b-pawn isn't weak because it may move when it likes to. The white b-pawns may become targets, but they're not on a half-open file yet. The b2- pawn is the weakest of the b-pawns because it can't be defended by another pawn.
In this position the d6-pawn is called a backward pawn.
It has some disadvantages: it can't be defended by another pawn, and it can't easily be moved out of the way. When it sits on a half open file, it can become an easy target. Most of the time, the square in front of a backward pawn will make an excellent outpost to the opponents pieces (especially the knights).
The backward pawn may have an advantage as well: it supports the more advanced pawn next to it. This may make this advanced pawn strong.
To get rid of your backward pawn, you'll have to advance it at at the right time. This may open some useful lines and diagonals.
In this position, black's c-pawns and white's f-pawns are doubled.
The disadvantage: Doubled pawns are not very flexible. They may be weak and undefendable.
They have a clear advantage as well: doubling of pawns always opens a file. When you have doubled pawns, you'll have at least a half open file as well. This half open file can be valuable and may be used by your pieces. Doubled pawns may give you some extra influence on valuable central squares.
In this position White's e-pawn is isolated.
Disadvantage: The isolated pawn can only be defended by pieces and is very weak. Usually it can't easily advance. The square in front of an isolated pawn is often used to blockade the pawn.
Advantage: The isolated pawn may give you some space advantage. It may give support to your pieces in the enemy territory as well.
Here the black c- and d-pawn are known as hanging pawns.
Disadvantage: Hanging pawns don't defend each other. They're vulnarable to an attack. As soon as one of them moves forward, the other one becomes backward and weak.
Advantage: Hanging pawns usually control four squares in the enemy camp. This gives you a space advantage. You may use this space advantage to reposition your army and start an attack. At the right moment you can advance one of the hanging pawns. This opens lines and diagonals and maybe creates a passed pawn.
The opponent's weak pawns may become a target. During the middle game you may attack them with your pieces.
In the endgame you may win them with your king as well.
In most cases, it's not easy to win in only a few moves. The opponent may use his pieces to defend the weak pawns.
In this position, the weak pawns are well defended.
Usually it takes some maneuvring to create other weaknesses as well. After this is done, play against two weaknesses may win the game.
This is a well known position arising from the Sicilian.
Black (in this game Viswanathan Anand) has just played his knight from b8 to d7.
You may notice that Black has two backward pawns. The one on d6 will be the main target, as this one is located in the centre.
1. Rfd1, Qc7;
Do you see the ideas? White is putting pressure on d6, while Black is defending d6 and tries to create pressure along the c-file.
2.Bg5, Rfc8; 3. Ne1!
The pressure slowly increases, so White overprotects the c2-square. As soon as the c-file is sufficiently protected, White can make use of the backward pawn.
The square in front of the backward pawn is an excellent outpost for the knights. The only question is, how to get your knights to this juicy square.
3....Qb7; 4. Bxf6, Nxf6; 5. Nd5,
After some exchanges, the first knight has arrived! Black isn't happy with this, so he has to chop it off.
5...Nxd5; 6. Rxd5,
Do you see what White is doing? She's retaken with a piece and not with the pawn. With the piece, there's still pressure against the backward pawn.
If there would be a white pawn standing on d5, White wouldn't have pressure against the d6-pawn any longer. And what's important as well, the d5-square couldn't be used as an outpost (for the white pieces) anymore.
6....Rc5; 7. Rad1, Rxd5; 8.Rxd5, Rc8; 9.c3
You probably noticed that Anand tries to get rid of the white pieces. He has already swapped one pair of rooks and wants to exchange the second rook as well. He also threatened to play d6-d5 as soon as the white rook would vacate the d5-square.
All of this is now prevented by the c3-pawn. (9...Rc5; 10.Rd1, d5? 11.b4, attacking the rook andwinning the d-pawn).
9...b4; 10.c4, g6; 11. g3, Rc5; 12.Rd1, a5;
As you see, nothing has been decided yet. The black pawn is still backward, but there are no signs of White winning already. During the rest of the game, the battle will revolve around this weak pawn and the square in front of it. White will try to put the knight on d5, restricting blacks pieces. If this plan succeeds, White will be attacking and Black will be defending.
13.Nc2, Kg7; 14. Qd3, Rc6; 15 Ne3, Qc8;
White is making progress and the knight is ready to jump into d5. White also plays a lot of moves to improve the other pieces and prevent any counterplay.
16.Kg2, Qe6; 17. Qe2, Bd8; 18. Nd5,
This is the setup White dreamed about. The knight has reached a strong outpost and covers some important squares in Black's camp.
The rest of the game is about how to attack.
If you're interested in the rest of this game, go to play against weak pawns part two.