Black to move
This is a position from a recent blitz game where I played the Stonewall Attack. Black played 16…Bxe4? and lost without a fight. Exchanges are not always mandatory: 17.Qxe4 Qd5 18.Qh4 Bd8 19.Qh6 Be7 20.Rh3 1–0
In these types of positions black almost always has to play …f5 and here he missed the opportunity twice. His last chance to try and salvage something was with 18…f5 but even then after 18...f5 19.e4 Qxe4 20.Qxe4 fxe4 21.Bh6 his position is not very good.
In the diagram he should have played 16…f5 forking by B and Q which would have forced me to capture ep. 17.exf6 Bxe4 18.Qxe4 Bxg3 winning the exchange
Then after 19.Qxe6+ Kh8 20.hxg3 he should continue 20…Rxf6 (which is better than 20...Qxf6 21.Qxf6+ (21.Qxc4?? is a blunder because after 21...Qf2+ 22.Kh2 Rf5 Black has a winning attack.) 21...Rxf6 22.e4 with an unclear position) 21.Qxc4 with an unbalanced position that is anybody’s game.
Remember that in these types of positions you almost always have to play …f5