Fredthebear wrote the following advice for another collection, but decided to stick it in here for the time being because it has everything and yet nothing to do with the Three Knights and just leave it be for awhile...
Years ago I read a good book on practical chess openings (I think that was the actual name of the book!) written by the dry but far-sighted, technically accurate GM Edmar Mednis, who also wrote a book about beating Bobby Fischer. Mednis suggested that club players defend against 1.e4 by the Caro-Kann, Petrov Defense, OR the Sicilian Dragon -- pick one. While I'm not one to disagree with GM Mednis, that is a very diverse range of offerings.
Do not fret over your defensive choice! Play the Black defense that you like, and learn it well. It does not matter what someone else says is the best defense. There is no silver bullet opening, no Black magic defense -- you must study to prepare it for all the possibilities that arise. If a grand master has played your choice of defense in a world championship match, then you can be sure it is sound and worth playing. (Oh do take GM Danny King's advice and leave your f-pawn on the f7 square. Your king desperately needs his f7 pawn to stay put there. Of course, there are exceptions if you have studied a specific line.)
It is vital to follow general opening principles, especially as Black having made one less move. Remember to develop ALL your minor pieces quickly, especially the king's knight to the f-file. The knights, bishops and center pawns do the early fighting. Move a pawn to let a bishop out; otherwise you probably should not move that pawn if it does not help the bishop in some way or protect something. Fight for control of the center (by occupying or aiming into the center with multiple units), and race to get castled. Castling is a crucial safety move for your king and a development move for your rook. Tuck your king toward the corner so he's not stuck in the boiling battle about to rage in the center. A rook's first move is generally sideways toward the center to support it from behind. Do not keep moving the same unit over and over. Try to move five different units in the first five moves, although it is fine for a center pawn to move twice to capture (or recapture) another pawn threatening to capture it. Then return to developing ALL your minor pieces rapidly.
Do not go chasing after pawns with your queen. It is dangerous for your queen to move twice to capture a mere pawn, but it's usually OK to move a queen twice to check, check, check or capture an undefended piece. (Technically speaking, pawns are NOT pieces. There are 8 pawns and 8 pieces starting on the back row. I use the term "units" when referring to any of army of 16.) If the queen can advance to give check and simultaneously fork an undefended minor piece for capture next, do so by all means. What I'm trying to say is that there is a time and place to bring the queen out early -- know when and when not to bring her out.
Be careful not to align your queen on the same open line as your king. A king and queen on the same line is easily pinned, skewered or forked with check by one opposing unit. Royalty makes an excellent target (study Paul Morphy games to watch him murder kings and queens). Realize that your king has the double duty of guarding f2/f7 and the queen. He can be forced by check to flee from her. That's why it's better to castle and let the rook (or minor piece) protect your queen.
Protect your c2/c7 and f2/f7 pawns from the glare of the opposing bishop and the intruding knight. Your queen is guarding the c2/c7 square; she might need some help with that by Na3/Na6 as a second defender or a3/a6 to stop the opposing Nb4/Nb5 advance. Otherwise opening moves along the rook's file are usually bad because it has nothing to do with the center. Fight for control of the center, and prevent penetration!
The center of the chess board is the military high ground; it's almost like playing the rough and tumble childhood game called King of the Hill. Your pieces have more scope in the center to attack or defend in any direction needed. For example, a knight in or near the sweet center attacks 8 different squares, but a knight in the corner attacks just 2 squares and is vulnerable to getting caught there. Centralize your knights to enhance their powers!
Play with your pieces, not pawns! Make more moves with your pieces than with your pawns. Your pieces cover more ground and are more likely to make checks and captures. Pawns are slow and can't return back to home base like a piece can. Pawns prefer to stay back and shield or support pieces. Pieces do not like to sit still guarding (babysitting) a pawn that ran off down the board; pieces prefer to be free to fly around (out-on-the-town).
Obviously, advanced players reading this already know such information but it might help to remind your chess students of such basic concepts they are violating. As always, accurate analysis prevails, not general rules of thumb. One of the best times to violate a chess principle is after the opponent has done so to take advantage of the violation.
Do learn the theoretically correct opening lines and the traps to your chosen defense. Memorization is a MUST!!! (I carry along a small 5x3" pocket notebook of my selected opening lines wherever I go and glance at them during stops, delays and breaks. If my duty is monotonous and requires little or no focus, I visualize the opening lines in my head like blindfold chess to pass the time of day. That's a true test of memorization.) Then use a chessboard to go over at least 50 master games, including long games (40+ moves) to get a feel for how the opening transitions to the middlegame and the endgame. Replaying master games in your chosen openings is a MUST!!! One thing leads to another and another.
Once you've invested hours, days and months learning a defense, keep that particular defense in your repertoire always. If nothing else play it as a second-string defense for casual games at your local chess club or the last round of a tournament when you're out of the money. (I usually begin a rated tournament with my second-string defense against the lower rated player in the first round and save my preferred defense for the best players.) However, it's perfectly fine to have only ONE defense to 1.e4 and play it all the time.
If you play chess for a lifetime, you will face 1.e4 tens of thousands of times, so having two different Black defenses prepared provides some spice. Do include GM move variations/branches within those particular defenses. Realize that your local chess sparring partners will appreciate you for playing more than one defense to 1.e4 from time to time. You bore them if you trot out the same one worn-out opening all the time, year after year. You risk losing your sparring partners from lack of interest. If it's a friendly game of casual chess, play something you do not ordinarily play and stick to good opening principles mentioned above.
I purposely play unpopular defenses at our club just to keep interest up. Alekhine's Defense has come in handy for this reason. Lower-rated players of the White pieces get easy development and feel good about their start with chances to mix it up; it keeps me on my toes about how best to undermine those extended pawns. However, I learned not to play foolish openings like 1.h4 because my fellow club members are embarrassed and discouraged after losing to 1.h4, which is not good for maintaining our club membership enrollment. Furthermore, I don't want someone to start emulating me playing 1.h4. When it comes to an officially rated tournament game with slow time controls, I am disciplined to play either of my two best defenses.
Do not blame the opening when you lose -- especially if you lost quickly. Simply learn what mistake you made in the opening (look up the proper theoretical book move which you should have memorized correctly) and play it better the next time! If you do not repeat the same mistakes, you'll start to have success with your chosen defense. Chess mastery is the elimination of mistakes. CHESS MASTERY IS THE ELIMINATION OF MISTAKES. Record and study your games; don't make the same mistakes over and over!
Finally, chess is way, way, way more than just getting off to a good opening start. There are three phases to a hard-fought chess game... the opening, middlegame and endgame. (Checkmate can occur in any of the three phases; the endgame is not another word for checkmate.) The opening lasts through castling and nearly complete piece development. The middlegame focuses on tactics and strategy; coordinating your pieces at select targets to gain material and inflict permanent weaknesses while avoiding the same from happening to you. The endgame occurs when most of the units have been exchanged off the board, the threat of checkmate appears to be over and the focus shifts to hurrying to promoting a pawn to make a new queen for an overwhelming new advantage. The king, no longer in danger of checkmate, awakens from his corner spot and becomes a fighting piece in the endgame.
Have you read the same endgame book three or five times through? Do you recognize the solution to the diagrams right away? You'd better if you play the Caro-Kann defense.
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Thank you delkin!