“Pawns cannot move backwards” we are told as a reminder to consider our Pawn advances with the utmost care.
We should give just as much care to our exchanges, they often favour one side, even if only slightly – but they can also be the critical point of the game. For instance, a Queen exchange that moves a middlegame advantage into a lost endgame – well, that’s certainly a turning point.
In the position below, I had to decide whether to exchange Queens or not and poor time management meant I only had a few seconds to decide. I swapped off with …Qxe1+ and went from a winning position to a lost endgame (the bad g7 Bishop versus the good e3 Bishop being the deciding factor). It pays to think of these “what ifs?” based on pawn structure earlier in the game and reassess as and when pawns move or are exchanged.
There are general advices that work well, such as “when ahead on material, exchange pieces, not pawns” (and the flipside, “when behind on material, exchange pawns, not pieces”).
Rook and pawn endgames are usually more likely to result in a drawn game than a favourable King and Pawn endgame for instance.
So, if you reach a point in your game where you have the option of trading pieces, especially if the exchange takes you to an endgame or a simpler endgame, consider who is favoured by the position when those pieces are off.
If you cannot assess the resulting position decisively then do not exchange! You can usually exchange later if need be, but once the pieces are off the board, the decision is final.