One of the only things we can't take back are pawn moves. While moving pawns forward usually provide us with space, they can also leave permanent weakensses in our position. Even the most innocent pawn move like h3 can create big weaknesses around our king.
Of course I am not saying to never move pawns at all, but it is imporant to be careful and know when to advance. For example, look at this position:
I have played both sides of this position and white's idea is simple: prepare e4. Now this is a position where white usually opts for a pawn minority attack, but f3 prepares e4 and covers the strong e4 square for black. White usually plays Rd1 and tries to cover important squares (like g4) before playing e4. He can play e4 in this exact position, but I believe black can equalize easily.
Now let's look at a game I played (as black) where white ( he is close to 2200 Fide)played e4 in this position. How do you take advantage of that move?
This is why I do not like the move e4 without preparation. so the e3 square is now weak, but it seems like an impossible square to take advantage on (e.g put a knight there). Let's look at some more moves to see how the game continued.
Even though white weakened his king a little, his position looks promising: He has an open f file where his rooks are putting pressure on f6 and f7. While my position seems solid, I need to do something quick to start putting pressure on white. What should black do next?
So after for example R3f2, what is black's plan? My idea was to push the queen side pawns and hope that my flank passed pawn proves to be stronger than white's D5 passed pawn. My opponent didn't want to sit and wait for me to promote a new Queen so he decided to sacrifice.
I had this position in my head when I played Kh8. I did not want to defend and prove that sacrificing the exchange was bad. Instead I wanted to attack his king. I knew this position was around equal after the precise Rf2! which gives the h3 pawn, but stops the attack. It is a very difficult move to make, but it was the best.
Black's attack came so fast and naturally, but why? If white didn't have that pawn on h3 and instead had it on h2! h3 not only allowed me to snatch that pawn, but it also gave me the extremely powerful g3 square. Of course the move h3 was not the reason why white lost the game. He made several mistakes throughout the game. The early e4, the clumsy move a3, the Rxf6 sacrifice and the last but biggest mistake Kh1??; however, my position would not have been as strong if I didn't have strong squares like e5 or g3 to use for my attack.
Back to the game, how can black punish white if he takes the bishop?
After Kg1, the game is not over yet!!
Let's look at some more moves.
The game went on for several more moves. I was overcome with pride and played carelessly for many moves. When the position got difficult to win I decided to take a long think and finish the job.
I know most of my readers have seen and read many games here on chess.com about weak squares. But I am telling you that is not enough to master those ideas. It is very important to put these ideas in practice. What I did when I was a B player to improve this skills was to see all of my opponent weaknesses, even the ones that didn't seem exploitable. Every time he made a move, I would think to myself "great, more weak squares for me to take advantage of." It is impossible to take advantage of weak squares in all games (if I am gonna be checkmated I am not gonna try to put a knight on a pretty square, I am going to use it to defend my king.) It is impossible to not create a weakness, and it is important to know when to do it. We will have to leave that for another day.
Here is the full game ( well, most of it) for you to enjoy.