Trying to follow opening principles
I saw this question pop up on twitter, and as someone who loves the opening phase in chess and the person posting the question, I thought I'd just shoot them a quick word of advice about the game. Opened up the game, and saw it was an interesting and tough opening, but I definitely had some advice on the subject. It ended up not fitting in a tweet...
Here is what the white player had to say:
"I'm a d4 player. Played e4 today against a stronger player, no prep, just tried to respect basic principles. Have several questions, such as 9. b4 (thought I was being clever preventing the knight from occupying c5).
Definitely thought of an early h4 but had sort of decided to play a slow game once he played 1. ...d6, which played a role in eliminating h4. This kind of lack of flexibility in mindset is an issue with us lower rated players."
Here is the game (variations are not mine).
Here are my abbreviated thoughts about this (there's tons more to say):
You could get this kind of position with 1.e4 or d4. It's not enough for your moves to respect opening principles, they also have to take into account where the opponent is playing, and often how you are planning to respond to different options they have. There is not always a principled reason why you put a knight on c3 v d2; it could be based on whether the opponent has a pawn on c6 or it could be based on needing to play c2-c3 to defend your d4 pawn against one option they have.
After all, what are opening principles? How did they tell you to play Nc3 and a2-a3 in this game?
Even more so than in other openings perhaps, in the modern, you need to be prepared for many options from the opponent. They could suddenly play e5, and you have to have in mind a way to use your position in a KID or Ruy structure; they could go e6-d5, and you'd have to have a way to play a French; they could go c5, and you have to be prepared for a Benoni or Sicilian (your choice).
In this particular game, your opponent chose a6-b5, which most often is followed up by c5 (partly because that will build consistent strength/ a head-duo in one area of the board). So you needed to have in mind whether you wanted a Benoni or a Dragodorf Sicilian. Your other option is to prepare c2-c3 by going Ne2 (there's other rarer options like answering b5 with a4 b4 Nd5 or ramming the h-pawn as you mention). With the development scheme you chose-- Be3, Bd3-- white is not excitingly placed for either the Benoni or Sicilian. One consideration is that in general, the Benoni is a worse structure for black than the Sicilian, so all else being equal, you might want to aim to be ready to answer c5 with d5.
Here are a few specific ideas that hopefully illustrate my approach to thinking about this opening:
* If you respect black's q-side play, answer a6 with a4, predicting the whole Benoni/Sicilian transpositions: that would be a normal trade-off of moves in those openings that slows or prevents black's counterplay. You are not really losing much with a4, you secure the c4 square for your pieces in some cases, and since they played a6 you are not falling behind in development.
* If you think a6-b5 is too fancy, and black should not be able to spend that much time setting up a positional advantage, then either get ready to trade pawns by racing ahead in development. a6 Bc4!? (right into their tempo move) b5 Bb3 Bb7 O-O. If they go for c5, you're trading. You may threaten Bf7+ soon, so they probably have to go e6 to block in the bishop. You can play d5 forcing e5, or stay flexible developing with Re1, but don't let black play c5 at a good time, and make space for your bishop with a4 in timely fashion.
* OR prepare to answer b5 with a4 trying to prove that black is not well enough developed, and so their pawn pushes will cause weaknesses rather than building strength. The downside is your c3-knight will be kicked, so e4 will be tender. Thus, it's logical to play Bd3 before Be3 if this is your plan: 5.Bd3 b5 6.a4 b4 7.Nd5 and black's b4 pawn doesn't look so hot. (c5 dxc dxc Bf4 highlights how development lead has an effect on whether space is strength or weakness.
* If you think a6-b5 is a reasonable (not obviously good or bad) use of time, and want to mitigate its impact by developing reasonably quickly, breaking up the head-duo, and preparing to support the d4 pawn, then you can go for an idea like 5.Be3 b5 6.Bd3 Bb7 7.a4 b4 8.Ne2 (Or Nb1-d2 is similar) Nf6 9.Ng3 c5 10.c3. Black will be able to open up some space for counterplay on the queenside, but there is no head-duo bullying your pieces, and you have supported your center at d4, so you still have a nice strong duo. This might be white's choice if you are afraid that 5.Bd3 leaves d4 too weak for a moment against Nc6/Bg4 for black. (e.g. you don't like 5.Bd3 Bg4 6.Be3 Nc6 7.Be2 e5).
Remember that because of black's flexibility, each development move has to be prepared to deal with a variety of different angles black can take against your center. 5.Bd3 might look good against b5, the move that a6 suggests, but you still have to make sure that it's not caught flat-footed by d5, e5, c5, Nc6, etc. This is why many people find it quite difficult to play against the Modern Defenses.
* With a3, you are neither getting ahead on time, nor preventing black from setting up their head duo with c5/b5. Your position will lack punch in either a Sicilian or Benoni.
* b4 weakens the c-file, which is black's major area of play in the Sicilian, so it's quite an anti-positional move. I suppose your position was not great at that point, and you were afraid that they would play Nc5 and get the bishop pair. You have to accept that white's position is already a bit awkward and choose the lesser evil, which I think here is d4-d5. Yes, c5-c4 and b5-b4 are hanging over you, so your c3+d3 pieces are badly placed (security is one of the 3 main measures of a piece's placement). But you have blocked out Bb7, and avoided any major concession. If black plays e6, you'll generally be able to trade and get a complex position where black's more flexible pieces are balanced by their pawn weaknesses on d6/e6. A continuation like 9.d5 Ngf6 10.O-O O-O 11.Rfe1 and then Rad1, and your center is defensible. (and in case of c4 you have the important f1 square for that awkward bishop).
