why 2. c4?

so i've been studying lately, reading through a book called Chess Openings for Black, Explained, by GM Alburt and GM Dzindzichashvili, and I've sort of chosen which openings i will use as black to defend against 1. d4. however, i've hit a wall in that most of my games end up looking nothing like book games (duh), partially because when i start off trying to play the nimzo or bogo, white almost never seems to play 2. c4 after i play ... 1. Nf6. it's been hard to put my learnings into practice when no one will play against me thew way the book tells me they will!

I'm to the point in my studies in which i understand why white would play 2. c4 to maintain an advantage in the center, but if they don't play it, i don't know how to take advantage of that.

in my book, Alburt writes, "Gms generally agree that to try for any advantage, White must now play 2. c4."

I know that is a good move, but why would GMs think that it's the only move? And if it truly is the ONLY move, then how do I take advantage of it when my opponent follows 1. d4 Nf6  with 2. Nf3  or 2. Nc3?

please, someone who knows the theory better than I do, HELP!

That's too strict a judgement. Some people are too categorical, e.g. claiming that other first move than 1.e4 is a mistake, etc.

2.Nf3 in this line is perfectly ok. 2.Nc3 prevents 2.c4 (which is a good move played to control the center), so, objectively speaking, it's a bit weaker than 2.Nf3. However, the difference is minimal, and I know many masters who play 2.Nc3

The book you're reading is probably exaggerating. While 2. c4 is the most popular reply, it's certainly not the only sound one; though as a devout Queen's Gambit player I always follow up with that move. In any case, other possible second moves for white are 2.Nf3 and 2.Bg5, both of which are perfectly playable. To follow up 2.Nf3, Black can try responding 2. ...d5 transposing to a closed game (1.d4 d5) opening. For 2.Bg5, it's quite a bit rarer but possible black responses (assuming you intend to avoid doubled pawns) include 2. ...Ne4 (attacking the bishop and forcing a retreat or defence) and 2. ...e6 (letting the queen defend the knight).

EDIT: Come to think of it, the fact that you're playing 1. ...Nf6 probably means you're trying to avoid symmetrical queen's pawn openings, so I guess responding to 2.Nf3 with 2. ...d5 might not be too much to your taste. If you don't like 2. ...d5 I'm not too sure what moves would be good for you since I'm more familiar with symmetrical queen's pawn openings, although Wikipedia suggests 2. ...h6, 2. ...g6 or 2. ...e6.

I do not think 2. Nf3 is worse, but it limits white's (and black's) options in some ways. For example, white cannot play a nimzo-indian with f3 or Nge2 anymore. the same goes for the king's indian (that means no samisch) and in the grunfeld, putting the knight on f3 is by far not the only option (in fact Ne2 is somewhat more popular, if I remember correctly).
It is also limiting in the benoni; white's most aggressive systems against it do not involve an early Nf3.

White does gain extra options in playing some queen's pawn systems like the torre attack or colle system or london system.

On the other hand black cannot play a budapest gambit or albin's countergambit.

You seem to limit your studying of the opening to the book variations. You'd be surprised how little this matters (at least until you're at a very high standard) and its more the strategies and principles of the opening that are the important concepts to master. If someone plays a variation you haven't studied then you have to use this knowledge to take the advantage. Does it limited the opponent's options? Has it cramped their position and thus set them on the defensive and you the attack? You take c4 and Nf3 - c4 is commonly played to put a knight on c3 afterwards - allowing development of the knight and attacking of the centre. Nf3 on the other hand means white is being more cautious, defending their d pawn rather than attacking the d5 - can black take advantage and play an aggressive game?

There's lots of ways to practise or learn about variations - try looking up tournament games (I use www.365chess.com) or using chess computer opening training programs.

Hope you find what you're looking for!

The thing is, if white doesn't play 2c4 with the idea of a queen's pawn game other than queen's gambit, black can and should play c5 at the appropriate time. In my opinion black should seek to assume the role of first player, albeit, more guardly by this strategy than if he were white. Queen's pawn games demand a great deal of patience.

I have a copy of that book too and it gives variations to responses other than 2. c4 vs. 1...Nf6. To me, all other responses except c4 on move 2 or 3 against 1...Nf6 or 1...d5 are weaker.2. c4 gains queenside space and follows the principle that in closed or semi-closed games, you shouldnt block your c pawn with a piece.

If my opponent doesn't play 2. c4 and plays something like 2. Nf3 or 2. e3 instead, I like to go on the agressive side with something like 2...c5.

After 2. Nc3 white no longer has imminent c4 ideas so 2... d5 is by far the simplest and most logical counter also discouraging e4 ideas. If you play Nimzo/Bogo against 2. c4 you may want to meet 2. Nf3 with 2... e6. If then 3. c4 you can go for the Bogo and 3... c5 should be a decent move against 3. Bf4/Bg5/g3/e3.

In the book you mention: Chess openings for black, explained, check the chapters where white doesn't play 2. c4. They deal with the common white setups without c4: the torre, londen, colle etc. They give a very similar approach to each of these, which are very easy to learn. So check out those chapters in your book :)

2.c4 signals that you want to play grownup chess. 

2. c4 pressurises black's centre and keeps white's options open more effectively than other moves. If white is fine against all of black's continuations then 2. Nf3 is as strong. All other second moves for white are slightly less strong but only if black's positional sense is sufficiently developed to take advantage of them. Engine analysis would not pick up on the subjective aspects and, to a lesser degree, neither would statistical analysis.

If your opponents are not playing 2. c4 or if they don't maximise the central pressure by playing Bg5 in a QGD, then black should generally play c5. Black also has less problems developing the c8 bishop in lines with less central tension. Many relative newcomers don't play 2. c4 because they're unsure how to proceed against the many defences black can use and some of these lines are sharp, so they avoid them.