What are the middle game principles? I have already seen numerous principles regarding the opening and ending, but not the middle.
1 - Centralize and coordinate your pieces. This is when tactical opportunities will begin to materialize.
2 - When your position is cramped, exchange pieces.
3 - When ahead in material, exchange pieces, not pawns.
4 - When you have a bad or inactive piece, exchange it.
5 - Don't launch an attack on the flank until the centre is secure.
6 - When the opponent's king is exposed, look for tactics and combinations.
7 - When the opponent's development is poor, look for tactics and combinations.
8 - Attack your opponent's weak and backwards pawns.
9 - Occupy your opponent's weak squares. Knights are particularly good at establishing outposts.
10 - Don't make unnecessary pawn moves in front of your king. This weakens your position.
"When looking for tactics - for either player - look for Checks, Captures, and Threats, in that order - for both players."
*"If you are winning by a piece or more, THINK DEFENSE FIRST. That does not mean play passively."
"The Exchange (winning a rook for a bishop or a knight) is worth about half of a piece."
*"If you are interested in learning, think of a draw offer as an offer to remain ignorant!"
"Putting out your hand when you offer a draw is presumptuous; always put it out after the draw is agreed upon, not before."
"If you get way ahead, it is more important to use all your pieces and safeguard your King than it is to try and get further ahead."
"Most games between lower rated players are won or could be won on tactics, so studying tactics when you are lower rated is much more important than anything else."
"Attack pinned pieces with pieces worth less than them; never take a pinned piece unless it leads to some sort of tactic or you cannot maintain the pin."
"Having the 'Bishop Pair' - two Bishops when your opponent does not - is worth about half a pawn."
"Bishops are better in open positions - Knights are better in locked/closed positions - Bishops are also better when there is action on both sides of the board."
"When ahead pieces trade pieces, when behind pieces, trade pawns."
"If you have a pair of Rooks and your opponent only has one, it is usually correct to trade Rooks."
"Knights are 'Path' pieces. Have an idea of where you want one to go!"
"Patzer sees a check - gives a check," but "Always (consider) a check, it might be mate."
"Doubled pawns are not always weak."
"When capturing with pawns, it is correct most of the time to capture toward the center. If the result is doubled pawns, this is correct even a higher percentage of the time." It is also more likely correct when capturing on a Knight file (b or g).
"A premature attack is doomed to failure."
"You can't attack where you don't have an advantage."
"An attack on the flank is met by a counterattack in the center." Especially if the center is not fixed/stable.
"Rooks need open and semi-open files. Don't let your opponent control open files with his Rooks."
"Don't 'give up' control of an open file by trading Rooks and bringing your opponent's other Rook to the file unless you can either neutralize the file by moving a Rook/Queen to that file or have some tactic."
*"It is not enough to recognize tactics. You must recognize them quickly enough that you will see them - even without prior knowledge that they are there - during the short time you have to move in a normal game"
"If the four e and d pawns are all locked together, then the 'pointing rule' says 'put your hand across your d and e pawns so that it points toward your opponent (the other way hurts your hand!); whichever side of the board your hand is pointing is the side you should attack - and also the side you should play the pawn break on the c or f files'" Note: Don't apply this guideline unless all four pawns are locked against each other!
"When the game gets complicated and there are a lot of captures, it is usually correct to make a capture (or at least an extremely strong threat) to hold the initiative. To make a quiet move usually leaves your opponent the opportunity to take the initiative." In any case, specific analysis and not general principles are required to make the decision!
"If you have some sort of structural or minor material deficit, it is better not to trade and to play aggressively, or else your opponent's long-term advantage will tell in the late middlegame and endgame."
"When both Kings are exposed and the heavy pieces are still on the board, the initiative (or attack) is often worth everything – so you usually should keep checking (or capturing) if you can." In these positions sacrifices of material are often quite common as you cannot let your opponent start attacking with tempo.
"Talk to your pieces. See which ones have the best arguments for moving them next. Never ignore a piece completely!"
"When your opponent has weak pawns, trade a few pieces. Then the weakness of the pawns often becomes easier to exploit."
*"If a piece is attacked defended, but some of its defenders are removable, then you can't count them as defenders - so that piece may be possibly won with removal of the guard!"
When you have several checks, consider first:
Checks that bring additional pieces into the attack, or
Checks that bring a powerful piece (like the Queen) closer to the King to guard more squares.
"Material doesn't mean as much if you castle opposite sides and queens are on the board."
Attack pinned pieces with more pieces, especially pieces worth less than the pinned piece.
In complicated positions with multiple capturing possibilities for both sides, it is usually correct to make the first capture, if possible.
The worst types of pawns are vulnerable pawns (backward, isolated, etc.) on semi-open files. Alternatively, doubled pawns can sometimes be strong, so don't count all non-perfect pawn structures as "bad".
It is almost always worse to lose a pawn than to take on a pawn weakness.
When creating luft to prevent back-rank mates against an opponent with one bishop, clear the square the opposite color of the bishop
There are two types of sacrifices:
Those in which the sacrificer does not capture material. These sacrifices you can take or not, depending upon which is better.
Those in which the sacrificer captures material. These sacrifices you almost always must accept/capture if your opponent's previous capture puts you behind in material, because you will lose anyway. Obviously if you see a mate in three and otherwise you lose a pawn, you lose the pawn, but most times you just capture. For example, you almost always capture the classical bishop sacrifice Bxh7+.
When your opponent has a pawn, bishop, or knight on "knight 3" (b3/g3 for White and b6/g6 for Black) then it is often a possible plan to attack it with a pawn, usually R4-R5 (a5/a4 or h5/h4 for Black or a4/a5 or h4/5 for White), possibly B4-B5 (c5/c4 or f5/f4 or c4/c5 or f4/f5)
If one side is weak on a particular color complex, the other side has a bishop of that color, and the side with the weak squares does not have that color bishop, then that is a large advantage for the side with the bishop, and even larger (possibly a pawn or more) if queens are still on the board and there are lots of pawns.
In the middle game, you are seeking an advantage that will win you the game. This could take the form of 1) an attack against the enemy king 2) the gain of a pawn (or more) of material or 3) a positional advantage that will win you the end game, such as a rook on the "seventh" rank (second for Black) a knight in a "hole in d5, d6, e5 or e6 (d4,d4,e4, or e3 for Black), a "good" bshsop vs. a "bad" bishop, a "passed" pawn, or a weakness in the opponent's pawn structure such as a doubled or isolated pawn.