This final-round tournament game is an excellent illustration of how most new openings are learned: the hard way. Learning openings is not really about memorizing variations, it's much more about knowing the standard early middlegame plans and how to handle common tactical and positional themes.
In the Dutch Stonewall, one theme is the pawn exchange on d5, if White initiates it. The usual rule (retake with the e-pawn rather than the c-pawn) applies here, and as Black I get a comfortable game out of the opening. The next key move occurs in the early middlegame, on move 11, and is the main teaching point: the potential power of the White pin on the a2-g8 diagonal. Here I ignore it and it makes itself felt immediately, then also later in the game on move 21. There are other useful lessons and observations that I got out of analyzing this game, but now I'll be very sure to recognize that theme in the future.
[Event "?"]
[Site "?"]
[Date "????.??.??"]
[Round "?"]
[White "Class A"]
[Black "ChessAdmin"]
[Result "1-0"]
[ECO "A90"]
[Annotator "ChessAdmin/Komodo 13.2"]
[PlyCount "61"]
1. Nf3 d5 2. d4 c6 {this is a committal move, but I was planning to play the
Stonewall in any case.} 3. g3 e6 4. Bg2 f5 5. O-O Nf6 6. c4 Bd6 {we now have
the standard Modern Stonewall starting position from theory.} 7. Nc3 O-O 8.
cxd5 exd5 {the general rule for an exchange on d5 in the Stonewall is to
recapture with the e-pawn. This keeps the c-file blocked, allows Black to use
the e-file for the heavy pieces, and gives the Bc8 more potential scope.} 9.
Bg5 h6 {the direct approach.} (9... Qe8 $5) 10. Bxf6 Qxf6 11. Qb3 {this move
should set off an alert, both for the pressure on b7 and the presence of the
queen on the a2-g8 diagonal, which means the d-pawn is now pinned.} g5 $2 {
I thought about sidestepping with Kh8, but didn't understand the urgency of
doing so. The text move of course is what Black wants to play, but needs to
better prepare it.} (11... Re8 $5 {is an active way of addressing White's
threat to advance the e-pawn, while developing the rook. The king can
subsequently go to h8.}) 12. e4 $1 $16 {now White gets in this strong pawn
break for free, as the d-pawn is no longer controlling e4.} fxe4 13. Nxe4 {
now this knight, which was effectively doing nothing on c3, is quite strong.
White also has better prospects for controlling the e-file.} Qe6 {breaking the
pin} 14. Nxd6 {by eliminating the dark-square bishop, this weakens Black's
ability to defend the entire dark-square complex, including the central square
e5.} Qxd6 15. Rfe1 Qf6 {I thought for a long time here and couldn't find
anything better. The alternative would have been to start developing minor
pieces, but I assessed that the text move would pose White more practical
problems, given the pressure down the f-file.} (15... Nd7 {is what Komodo
prefers, trying to catch up on development.}) 16. Re2 Bg4 17. Re3 Nd7 {White
grabbing the pawn on b7 would be OK by me, giving me some more counterplay
with the undeveloped rook.} 18. h3 $6 {this is essentially prompting me to
play the best, equalizing move. Which of course I do not do.} (18. Qxb7 Rab8
19. Qa6 Rxb2 $14 {and now} 20. Qxa7 $6 Nb6 $11 {with full positional
compensation for the pawn, as the knight will go to the strong c4 outpost and
the rook on the 2nd rank is very good.}) (18. Rae1 $16) 18... Bf5 $2 {an
example of how a piece can look nice visually, in this case on the h7-b1
diagonal, but it is not in fact doing much useful there. It even blocks the
f-file pressure.} (18... Bxf3 $1 {an illustration of the power of piece
exchanges. This should be a rather obvious choice, getting rid of Black's
"bad" bishop - look at all the pawns on light squares - for White's good
knight.} 19. Bxf3 Nb6 $11) 19. Rae1 (19. g4 {would have pressed the advantage.}
Bg6 20. Qxb7 {now the Nd7 is hanging, so White can profitably take the pawn.}
Nb6 21. b3 $18) 19... g4 {pinning my hopes on active play rather than passive
defense with ...Rab8.} 20. Nh4 gxh3 $2 {this ignores my opponent's tactical
threats involving the d5 square, but in practical terms it does encourage his
next move, which gives away the advantage.} (20... Nb6) 21. Nxf5 $6 (21. Bxd5+
$1 {again we see the problems related to having the king remain on this
diagonal.} cxd5 $18 {and White now has the pleasant choice between the simple
Qxd5 or Rf3.}) 21... Qxf5 $11 22. Rf3 Qh5 23. g4 {I thought for a while here,
but could not calculate the best plan properly, already being stressed and
fatigued.} h2+ (23... Qh4 $5 {I seriously considered, but I was too wedded to
keeping the h-pawn. Komodo gives} 24. Bxh3 Rxf3 25. Qxf3 Rf8 {and White has
nothing better than to simplify into an ending.} 26. Qg3 Qxg3+ 27. fxg3 Nf6 $15
) 24. Kh1 Qh4 $6 {here I did not consider the capture on f3, because of the
threat to the queen; however, it is a dynamic (if complicated) way of dealing
with the position.} (24... Rxf3 25. Qxb7 Rb8 26. Qxb8+ Nxb8 27. gxh5 Rxf2 28.
Re8+ Rf8 {and White has a slight edge, but nothing more.}) 25. Qd3 {my
opponent prefers active threats on the kingside (Qg6+) to snatching the b-pawn.
} Rxf3 {this is still good and should be equal.} 26. Bxf3 Nf6 {this is OK, but
unnecessarily complicated.} (26... Nf8 $15 {nothing wrong with good defensive
play, taking away the g6 square from White.}) 27. Qg6+ Kf8 $4 {when
calculating this, I missed my opponent's next follow-up, which wins. Fatigue
dulled my ability to properly calculate the two sequences.} (27... Kh8 28. Re7
{I mistakenly thought won for White, but Black holds after} Rg8 $1) 28. Re6 $18
Ng8 29. Qf5+ Kg7 30. Rg6+ Kh7 31. Qf7+ 1-0