I'm not talking about just what the openings are called or what the move orders are, I mean the actual theory and reason behind the moves and why those instead of other moves. And when there are other good moves which create new lines/openings, what the difference in theory is when you choose one over another.

I can memorize things easily so I feel I can "learn" many lines. But I find myself staring at the Game Explorer sometimes and saying "Why did they choose this here instead of this". Most times the other choices are also in the game explorer at those times.

I've found a couple videos on this site that do a pretty good job at explaining many of the moves (typically never all of them though), but then it is a very specific line in a specific opening.

Maybe I just think too much and the answers are easier than I try to make them out to be.

For example:

So how does one really learn all of that?

If you are referring to the "why" behind certain moves, I can tell you what I have been doing.  Asking!

Simply play top rated players, and ask them WHY they made a certain move, or why specific lines are better than others.  You will learn a lot simply by asking people.

I'm like you, I don't want someone telling me they played Be7 because it's "the main line", or "it has a high win percentage"...I look for people to explain what it accomplishes, i.e., an attack, a defense of several squares, and multi-stepped plan that leads to an attack, defense, or some other tactic.

I too ask the why on a lot of the 'book' moves, but I suppose the masters who wrote the book were often playing with the bigger picture in mind of the endgame, while most of us are looking to merely survive the middle-game and hopefully get a quick mate in. But in your example of the Sicilian I noticed you asked why should white play 2. Nf3 instead of Nc3 defending the pawn. White can play Nc3, and I will be doing so more frequently. I had a couple wins with white using it, but I couldn't find them, so I'll give you this draw I had at least to give you some idea of how it changes the game for black. I'm not an expert player, both me & my opponent made mistakes, some worse than others.

http://www.chess.com/echess/game.html?id=52474557

I annotated it without any program help, and I'm sure there's probably a bunch of inaccuracies I overlooked, so if anyone fancies pointing any of them out to me, that'd be more than welcome.

As far as your wondering about the mystifying purpose of the e6 push, I expect because usually the king's bishop is fianchettoed, and so the king's knight will often develop to e7, so as not to get in front of the bishop.

You are asking good questions, and your efforts to answer them will make you a stronger player. Learning the openings can be tricky: You don't want to blindly play memorized moves without understanding them, but opening theory is nothing more than the distilled experience of generations of good players, so it doesn't make a lot of sense to ignore it. Lets lok as some of your specific questions:

Why not 2.Nc3? Masters do play 2.Nc3 sometimes, they also play 2.c3. There's nothng wrong with either move, but most players prefer the kind of positions that come about after 2.Nf3 and 3.d4. These tend to be sharper and more tactical.

2...e6 is not as common as 2...d6 or 2...Nc6, but it does get played often enough. Black can pick from a few different systems of development: the Kan, after 3.d4 cxd4 4.Nxd4 a6; the Taimanov where Black develops his Knights on C6 and e7; or the line in the game, which could also have been reached after 2...d6.

Yes, both Black and White understand that with 3.d4 White is giving Black a central majority, while White gets a bit more space and quicker development. Apparently it's a pretty even tradeoff, since the best players seem willing to take either side of the deal.

4...e5 looks a bit risky since it leaves Black with a backward d-pawn, and a very weak hold on his d5 square. Whit'es Bishop could also be very powerful on c4, with nothing between it and f7. There are systems where Black plays an early e5 and kicks White's knight around, but after 2---e6, e5 looks like a loss of time.

Why not 5.Bd3 to open up castling? First, is White entirely sure he wants to castle on the king side? It's not rare for White to castle queen side in the Sicilian and push his kingside pawns. Second, is d3 really where you want to put your Bishop? C4 looks like a nice aggressive post, and on e2 the bishop will prevent any annoying Ng4 later, and it can go to f3 later and exert pressure against Black's queenside.

6...a6 not only takes the b5 square away, preventing any Nb5 attacking the d6 pawn later, it also prepares for futer queenside play for Black with b5, possibly Bb7 or possibly b4, ousting White's Knight.

7...Qc7 puts the Queen on a spot where is supports d6 and e6 and puts some pressure down the c-file. If Black plays b5, the Queen will protect the knight if Black plays Nc6.

It's a good idea to try to understand these moves, but don't obsess over it. It's not as if you're walking a tightrope of "only" moves. There are alternatives at almost every turn that are also fine  

Sometimes you really are walking a tightrope of "only" moves.

I think OP should consider getting a book or two and reading the verbal descriptions in it.

OP you say you find it easy to memorize lines, that's good!! I wish I had your problem. I warn you though that some of the opening books get into a ridiculous amount of variations. The Move by Move or Starting Out books are probably better for people starting out IMO.    

You are looking at the wrong direction.Middle-game and endgame you must study and it is sad to see good players "feeding" your confusion with useless opening information instead of guiding you to the right direction.

Have you studied endgame?Have you studied middle game?How many games with isolated pawn have you SERIOUSLY studied(not just read but try to analyse)?How many rook endgames have you SERIOUSLY studied?How many books with games from great players have you SERIOUSLY studied?Do you know what "minority attack" is?How many games with backward pawn have you SERIOUSLY studied?Can you evaluate a simple  position? Can you find a simple correct plan?

Don't answer me , I don't need an answer , your questions are all the answers I need.

Answer to yourself.

p.s. Keep memorising lines , in 2 years when your improvement will hit a wall just change the opening explorer or the database you use.It's their fault. 

I really have to disagree. Of course just memorizing opening moves is a waste of time, but learning the ideas and plans behind the chess openings is a perfectly valid part of building up your chess strength. Take those IQP positions, they don't appear on the board by magic. They're the natural outcome of certain opening systems, and almost never occur in others. The same would be true of the doubled c-pawn complex. They arise naturally out certain lines in the Nimzo and the Winawer French. The openings and the middle game are inextricably linked. 

I do agree that opening study should be a relatively small part of your chess studies, at least until you're pretty strong. Well annotated Master games are probably the best single source of chess education, but it's important to look at books aimed at the right level. "50 Essential Chess Lessons" by Steve Giddens or "Chess: The Art of Logical Thinking" by Neal McDonald are two good choices. But you have to be prepared to work at those games, not just casually play them over.

And study the endgame. There are two reasons why this is important. (1) If you can't cash in a winning end game, or defend a drawn one, a lot of your chess work will be in vain. What's the point of outplaying your opponent and winning a pawn, just to draw--or even lose!--the end game.  (2) Endgames can be incredibly beautiful and surprising. I've been trying to work on my own lousy endgame play recently, and I'm constantly being amazed by the subtle and  surprising twists and turns you can find, even in the most routine looking positions.  Get a good endgame book. A lot of people swear by "Silman's Endgame Course," but another good choice would be "Understanding Chess Endgames," by John Nunn. 

And most important: Play! You can't get stronger without constantly putting your new knowlege to the test and refining your understanding. There's nothing like a good, sharp defeat to show you where you need improvement.

IQP is not the only possible isolated pawn position.There can be isolated pawn from every opening.Referring to IQP as the only isolated pawn is either a superficial or just simply a bad case of an  example.

Information like:

"2...e6 is not as common as 2...d6 or 2...Nc6, but it does get played often enough. Black can pick from a few different systems of development: the Kan, after 3.d4 cxd4 4.Nxd4 a6; the Taimanov where Black develops his Knights on C6 and e7; or the line in the game, which could also have been reached after 2...d6."

is useless for anyone that hasn't done any serious middle game and endgame study , only increases his confusion and makes him think that is the important.

    You want to "feed" his confusion?Be my guest.I will even stop tracking that topic and let you do your job undisturbed.

good bye

My favorite book for explaining opening moves, and I have several, is still the FCO (Fundamental Chess Openings by Paul van der Sterren).

Some examples of his move by move explainations:

1.e4 c5 - "Like 1...e5, 1....c5 prevents the formation of an ideal pawn-centre with 2)d4, but without giving White the typical targets of the 1)e4 e5 openings..."

Open Sicilian 1.e4 c5 Nf3 - "White intends to play 3)d4, opening the d-file and taking charge of the centre, not with two pawns on e4 and d4 as in the Alpin but with a pawn and knight and - most importantly - with beautiful open lines and natural squares for all pieces."

Regarding 6)Be2, there are other options to the main line.

6.Bg5 - "This is often called the Rauzer Attack, reflecting the fact that the modern handling of this line, with Qd2 and 0-0-0, was devised by Vsevolod Rauzer. The fact that 6)Bg5 is more aggressive and forcing than 6)Be2 can be felt immediately."

Another thing that helped me with understanding opening moves was the pawn structure teaching of Danny Rensch in his videos. Basically, players who study openings are looking very closely at what pawn structures they will get after the opening phase, and learning how to exploit the strengths and weaknesses of all of those different pawn structures, i.e., do they want to get into a Scheveningen, Dragon, or Najdorf pawn structure. That's how they see chess, looking at the pawns on the board.

Now, some players with the White pieces may want to avoid all of those pawn structures above by playing the Closed Sicilian, directing Black, sometimes, away from his comfort zone, i.e. the Dragon.

The Richter-Rauzer is a line in the Classical, not the Scheveningen. 6. Bg5 against the Scheveningen is pretty tame by comparison. 

Conversely, 6. Be2 in the Classical is known to be completely sterile after Boleslavsky's 6. ...e5.

There is something you are completely missing, I play chess to have fun. If I find studying openings and learning the reasons behind the moves fun, that is what I will do. It is really that simple.

As for why you feel the need to tell other people that are enjoying a hobby incorrectly in your view, you may want to take some time to do some self inflection and see why that is. Your post did not add anything to the topic at hand. A topic that is about openings in the opening forum. You are welcome to approach chess and study/play it any way you wish just as everyone else is and you are just as welcome to let others do so.

Study long,

study wrong.

Just finish the game before there is an endgame!

Most opening moves is to develop pieces into their ideal position. I think what u should do is to study the middle game that arises from the opening. If u like the sicillian, u should read some opening books on the open sicillian and look out for their middle game plans and strategy for both sides.That should make u understand why some moves are played. Hope i helped! :)

I have considered some of the move by move books. My biggest complaint with most chess books is they tend to have a big paragraph of notation followed by a couple lines explaining just one point in the whole list of moves. And although I get that a lot of chess players have no problems visualizing a big long line of notation, I find it slow to do and slower to learn from.

Maybe the move by move books are a lot better about this by giving a lot of position diagrams to look at while reading the discussion about the moves. If so I will probably pick one of them up and go through it a couple of times.

Thank you for all the useful feedback and discussions in the thread so far.

Maybe u could set up a chess board while you are studying with chess books. This should make it easier!

Reuben Fine's "Ideas Behind the Chess Openings" although decades old is a good starting point.

The point of 2. Nf3 is to prepare to open the center with d4.

ChessSponge: if you own ChessBase (or even the free version of chessbase), you can buy many of Everyman's books in chessbase format, so you can play through every line easily. I love reading books in that format and wish more publishers would embrace it.

great discussion!  thanks for the read.  :)