What's the best way to play against this line? (in the QGA)

I switched to the Modern Benoni and found myself winning a very high proportion of my games. That was fifteen years ago or more. Now there's an heightened awareness of how to counter the Modern Benoni. I think I played my last Modern Benoni on Monday. I blundered and lost. I was very tired and probably tried to force the game unnaturally into lines that were too difficult but I also think I no longer have such a strong tactical ability. I intend to play the QGD for a while and see what happens there.

Black should be fine with draws, but I win many games with the QGA.

3. e4 is very strong for white and can be very tactical. I go with Nf6 for black. e5 is far too dangerous to be worth playing unless you really know all the dangerous lines.

But white never plays d5 too early, after which white's position is knackered, which is a technical term indicating reduction of opportunities.

I do not like 3.e4, because white has to be prepared against several good moves after it- e.g. so far I do not know a way to show some sort of an advantage after 3...Nc6.

I have looked here at all three possible white replies (Nc3,Nbd2,Bc4) but none of them seems able to trouble black.

3. e4 c5 is also a very difficult line. I play Nf3 e3 for the same reasons. I know where I am.

You'd be surprised how many players fall into all kinds of QGA traps as White.  Even strong players.  With that said, I don't rely on White to blunder.  The opening is extremely complex.  Equalizing can take a dozen or so moves easily, but from there Black has a very good game.  As it's my primary repertoire I try not to publish too much from it but eventually I'll be playing it in tournaments.

To return to topic though, could you still elaborate on why 3. ... e5 is a "bad" move for black (post #23)? How does white punish that move exactly?

Sorry I've been a while replying. I've been very busy.

I think that e5 for black allows the game to open up in positions where white has more space and activity and better development, and has various ways to sacrifice a pawn for the attack. So in correspondence chess or in engine assessments, black may be able to hold white but in practice against a well-prepared player, Black may have too much work to do and something will give. The Nf6 lines are more solid and tend to keep the position more closed.

When I started chess I played the e5 lines and sometimes got good wins, especially in the lines where black plays f5 also, but if black can play an early f5 with advantage then white has already gone wrong, more than likely. I switched after coming up against well-researched and very sharp methods that white can employ against e5. Alternatively, white can often achieve a better ending by various methods. Otb, black has to know so much to be prepared against white's possible lines.

being a 1. d4 player, I went through a phase of always playing 3. e4. I didn't know many book lines, if any, but I used to play Nbd7 to block Bb4+ and I found I got complex games where white always seems to have the resources to put black in trouble.

Perhaps you could on 2....dxc4 is play 3. e4 instead. The central Variation avoids 3...e5 and let's you finish developing. Ultimately it is hard for Black to keep his pawn he has gotten by accepting the Gambit. I see this line of 3. e3 played to swindle Black into trying to hold the pawn and open his h1-a8 diagonal to a well timed Qf3 to pick up the Rook or the Bishop.

It doesn't 3...e5 is the classic response to 3e4 too.  3.e3 e5 is by no means unusual and any decent book on the Queen's Gambit accepted. The article mentioned by IM Pfren is one of the best ways to learn white's ideas.

It's amazing how many people I play at Blitz around the 1700 rating mark try to hold the pawn. Most of the ones I lose, I lose on time. Maybe one in ten I blunder and get a bad position. Most of the time it's easy to counter by carefully selecting white's standard moves like a4, b3, Ne5, e4 etc.