It depends on the specific variation of the Queen's Gambit. You can trade on f6 for the knight if provoked by ...h6 in some lines, but again, it depends on the specific opening.
Queen's Gambit: dark squared bishop?
Hello. Yes, I'm sure it does, but I'm not yet mastering specific lines. Do I, for example, get the bishop out before playing e3? In which lines? Or do i move the bishop to the a3 or a1 diagonals? Thanks!
if e3 is played and bishop is still at c1, this is not queen gambit, but slav defence.
on QG either g5 or f4 are good, f4 less because there is less pressure on d5 making black game more easy.
if you play on g5, there is an line where after h7-h6 white takes on f6 and after Qc2 or Qd2 and 0-0-0, white (based on h6 weakness) has motive of g2-g4 and h2-h4 with strong attack, of coure this is double edge position since black will break on queenside and center very quicky too.
there is also possible to take on f6 under "normal" occasions where as white you can still have good game since your black bishop is always worse than blacks one (due to pawn structure) and black Nf6 is very important piece.
Hello. Thanks--very helpful! If you have an example game or two that you can share, I'd love to see a couple regular openings. I know there are lots of different lines and defenses, but I'm not that advanced yet. Also, is queenside castling the more common option for the Queen's gambit? Thanks!
Me again--another question. I also often have the Black opponent play bishop to guard the pawn on c4. What's a good counter to this? Thanks.

Usually if Bc1 is not taken out to f4 or g5 we do not call it queen gambit, does not matter about databases naming or what, it is slav position. still lots of slav positions may be called somethime as queen gambit, like botvinnik var when white goes for Bc1-g5, and not all the setups with black pawn on c6 are called slav, but QG...so do not take this names to seriusly, somethime border between queen gambit and slav is not so clear. and does not need. when i play, i am considered on moves and pieces not on names and ECO codes.

I like developing with Bb2. Usually after everything there's Nf3, Nd2, Bc4, Bb2 and Rc1... more of a general 1. d4 system than an opening per se. It works pretty well for me though, pushing e4 later and opening the center makes those bishops really shine.

At the intermediate level, what's the best option for developing the dark squared bishop? Where does it go to get developed, and is it good to trade for the knight, for example, on f6?
The question is too broad. Do U mean QGA or QGD, for example?

It just depends... e3 with the bishop still on c1, Bf4, Bg5 with capturing, Bg5 without capturing... all of these have been played by world champs.
What makes the move strong is whether or not you understand the resulting middlegame. Asking "what's the best opening move for an intermediate player?" This kind of question shows you misunderstand how these things work. This is not a useful or answerable question.
Yigor, good question: I'm thinking QGA, which seems to be what most of my opponents do.
The_Chin_of-Quinn, it seems like you are saying that there are different options for the black bishop, since you name different positions in your first paragraph, played by world champs. Is this correct? My underlying question was this: if I play e3 before getting the bishop out, is that a mistake. It seems like you are saying "not necessarily."
As to your second paragraph, doesn't the development come before the middle game, and doesn't the development of pieces determine one's position in the middle game? Aren't there openings that really require certain key moves in the opening, like, say, c5 for the Queen's gambit? It sounds like the Queen's Gambit has different options for the dark bishop.

Yigor, good question: I'm thinking QGA, which seems to be what most of my opponents do.
The_Chin_of-Quinn, it seems like you are saying that there are different options for the black bishop, since you name different positions in your first paragraph, played by world champs. Is this correct? My underlying question was this: if I play e3 before getting the bishop out, is that a mistake. It seems like you are saying "not necessarily."
As to your second paragraph, doesn't the development come before the middle game, and doesn't the development of pieces determine one's position in the middle game? Aren't there openings that really require certain key moves in the opening, like, say, c5 for the Queen's gambit? It sounds like the Queen's Gambit has different options for the dark bishop.
Yes, there are different options for the dark square bishop in the queen's gambit.
Yes, development comes before the middlegame. At first you should just develop and then afterwards look around for ideas, but with more experience, and as you get into choosing between opening variations (which is essentially what you're asking about here) you play opening moves based on the type of middlegame you understand or the type of middlegame you're interested in reaching.
For example the middlegame ramifications of Bf4 and then e3, is that your bishop influences white's kingside, the center, and black's queenside... and it is cut off from white's queenside. In some variations black has a natural pressure on the queenside dark squares as a result of that choice. It's not good or bad to play Bf4, it's just there are positives and negatives to every move.
In beginner-type manuals, they will often say something like e3 before moving the bishop is a mistake because it's locking the bishop in. That's true in general, but in the queen's gambit it's not a problem. It's just a different choice. Rarely will you be able to comfortably develop all 4 of your minors pieces (both knights and both bishops). It's normal to have 2 or 3 on ideal squares, 0 or 1 on an adequate square, and 1 that will need improvement later. In the queen's gambit a development like Nf3 and Nc3 are more or less ideal. Bd3 or Be2 is good or ideal, and the bishop on c1 is the piece you'll need to improve later... although as hinted earlier, it can be a useful to have it on your queenside to bolster the dark squares there.
Just to illustrate, here's an example of bad play by white where the bishop's absence on the queenside is quickly noticeable:
TCOQ: very helpful! I'm somewhere between the stages of "developing and then looking around for ideas" and developing with a strategic middle game in mind. But I'm surely not there yet. Your explanation of developing minor pieces and how the dark bishop fits in the QG is what I was looking for. I was under the impression that a good opening required all pieces to be completely developed, but maybe not. Thanks a lot.
TCOQ: interesting game--looks like one I'd play ;-( Qc8 followed by a 0-0 might get white out of the initial jam, right? Did the game end there? Thanks!

No problem. It helps if you have access to a database... you're a paying member so you can use stuff like this:
https://www.chess.com/explorer
https://www.chess.com/games
I use different stuff, so I'm not sure how to navigate those, but the idea is you want to play over whole games (between GMs) featuring the opening you're interested in, that way you can get a feel for the main middlegame ideas (like which side of the board a player plays on, and with pieces, pawns, or both). If you're only looking at move 5 it's hard (or impossible) to know what the difference between Bf4 and Bg5 is.
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I made the game up, it's not a real game.
edit - I'll delete the answer because you already posted it... yes, Rd8 works

Just some games to show how games are played with different bishop positions.
http://www.chessgames.com/perl/chessgame?gid=1124538 Bf4
http://www.chessgames.com/perl/chessgame?gid=1373778 Bf4
http://www.chessgames.com/perl/chessgame?gid=1396883 e3 before bishop develops
http://www.chessgames.com/perl/chessgame?gid=1359895 e3 before bishop develops
http://www.chessgames.com/perl/chessgame?gid=1129987 bishop stays on queen side
I prefer Bf4 as it has less theory and controls the e5 and b8 squares. I try to open the b file and prevent black's e5 lever, both are done with this bishop move.
I'm not familiar with this site's opening explorer, so I didn't look for games from here.
At the intermediate level, what's the best option for developing the dark squared bishop? Where does it go to get developed, and is it good to trade for the knight, for example, on f6?