2...e5, 2...e6, 2...g6, 2...Nc6, 2...c5, and 2...c6, 2...d5 and 2...b6.
2...e6, 2...g6 and 2...c5 should occupy most of your time. 2...c6 and 2...d5 transpose to things you would normally see after 1...d5. This leaves 2...Nc6, 2...e5 and 2...b6. I guess everything else is refuted.
In essence, you need answers to the following:
King's Indian Defense (2...g6 with an eventual ...d6)
Grunfeld (2...g6 with an eventual ...d5)
* NOTE: If you play the Fianchetto Variation, that would kill two birds with one stone. The Fianchetto Variation is 1.d4 Nf6 2.c4 g6 3.g3 (usually followed with Bg2, Nf3, O-O, and then it depends on what Black does
Nimzo-Indian (2...e6 3.Nc3 Bb4) OR you can play 3.Nf3 (allows Queen's Indian, 3...b6) OR Catalan (3.g3)
Benko/Benoni (2...c5 3.d5 and now 3...e5 is the Czech Benoni, 3...e6 is the Modern Benoni, 3...b5 is the Benko Gambit)
Slav (2...c6 and 3...d5) - This can also come from 1...d5.
QGD (2...e6 and 3...d5) - This can also come from 1...d5 and can also lead to a Catalan if White chooses.
What system do I personally employ? 3.Nf3 and 4.Qc2 against the Slav, Anti-Benoni (3.Nf3) against Modern Benoni Lines (will allow Old Benoni, 1...c5, then 2.d5 and I don't play c4), Fianchetto Variation against the KID and Grunfeld, Catalan against the QGD or Nimzo-Indian attempts.
Consolidated with post supra.
D-pawn openings are very transpositional, more so than e-pawn systems, and they often lead to lines that can start with 1. c4, 1. nf3, 1. g3, or seemingly unrelated moves. Black has numerous options as well: among them are the Nimzo-Indian (e6 with Bb4 against Nc3), the Queen's Indian (e6, Be7, b6, and Bb7), the King's Indian (g6 and Bg7), the Gruenfeld (Nf6 followed by d5) the Benoni (c5 meeting d5 with e5 or e6), the Old Indian (d6 with e5 or c5) and the Queen's Gambit with e6 followed by d5, a move order usually played before the f6 knight comes out.
G3 isn't a bad move at all to combine with c4 and forms the basis of the Catalan system. Sometimes the Bd3 development is preferred, but the Indian systems are reliable for both sides and need not be feared by either player. One trend that's picked up in recent years is players combining the King's Indian Defense against 1. d4 with the similar Dragon Sicilian formation (pawns on d6/g6, bishop on g7, knight on f6) against 1. e4. The Pirc/Modern defenses are structurally similar, so regard this as one big "family" of openings and read up on White's options against them. That being said, the restrained center (c4/d4 with the Bg2 fianchetto) is a fine defense against all of them. Castling is usually kingside, White may/may not aim for the e4 push, c3 is the b1 knight's natural square, and the dark bishop usually goes to b2 or g5 depending on Black's formation. Use this setup as a guideline and try experimenting with move order (i.e., starting with 1. g3 or 1. c4) if it helps to avoid unfavorable variations.
Thankyou all for being so helpful, exploring many of these ideas now.
You should play 2.Nf3 first for a few reasons:
1.Principle of flexibility, the knight will end up here anyway.
2.You sidestep the Budapest.
3.The pawn isn't committed to c4 in the case of the Benoni where fast piece development help more than the c4 pawn. A knight on c4 can also pressure a weak d6 pawn later.
2...Nf3 and Black sidesteped the Nge2 lines of the QGD exchange, Modern Benoni lines with an early f4 (1. d4 Nf6 2. Nf3 e6 3. c4 c5 4. d5 e6), and the entire Nimzo-Indian now you have to play the hybride lines, the QID or the Bogo-Indian.
2...Nf3 is not strickly superior to 2...c4.
Also, even though ThrillerFan already mentioned it, Black cannot treat the Nimzo and Queen's Indian as two seperate openings. They are seperate openings that Black can choose against White's 3.Nc3 or 3.Nf3.
In essence, you need answers to the following:
King's Indian Defense (2...g6 with an eventual ...d6)
Grunfeld (2...g6 with an eventual ...d5)
* NOTE: If you play the Fianchetto Variation, that would kill two birds with one stone. The Fianchetto Variation is 1.d4 Nf6 2.c4 g6 3.g3 (usually followed with Bg2, Nf3, O-O, and then it depends on what Black does
Nimzo-Indian (2...e6 3.Nc3 Bb4) OR you can play 3.Nf3 (allows Queen's Indian, 3...b6) OR Catalan (3.g3)
Benko/Benoni (2...c5 3.d5 and now 3...e5 is the Czech Benoni, 3...e6 is the Modern Benoni, 3...b5 is the Benko Gambit)
Slav (2...c6 and 3...d5) - This can also come from 1...d5.
QGD (2...e6 and 3...d5) - This can also come from 1...d5 and can also lead to a Catalan if White chooses.
LeEpitome, if you just starting to develop your repertorie, I would agree with TheGreatOogieBoogie's suggestion of 2. Nf3 for the sake of simplicity.
Something important to learn first is how to deal with the Queen's Gambit Accepted, particularly if black decides to try to hold on to the pawn. Yes, black holding on to the pawn is not the most sound, but this can be difficult to work out over the board.
@LeEpitome: Pay special attention to ThrillerFan's note. Playing g3 fianchetto systems kills a lot of birds with very few stones. You can meet 2...g6 and 2...e6 with the fianchetto.
Regarding the Catalan, be aware 1 d4 Nf6 2 c4 e6 3 g3 d5 4 Nf3 is the Indian route to the Catalan, while 1 d5 d5 2 c4 e6 3 Nf3 Nf6 4 g3 is also Catalan.
first post...
I have been playing a while now and have started developing my opening repertoire more. Generally I like to play 1. d4.
My main weakness with this is that after 1. ... Nf6 2. c4 I feel completely swamped the variety of different lines that follow on from here, to the extent that I have been opting for 2. g3 instead, I think that this is a disservice to myself as c4 is a very strong move. So, my question, what are the responses to 2. c4 from black? which can be refuted, and which demand attention? What system do you usually employ when playing 1. d4 Nf6 2. c4 as white?