"Rapid: 1135 ratings (hi 1221), ~600games"
++ This is a clear sign that you need to prevent blunders. Always check your intended move is no blunder before you play it. That alone is enough to get to 1500.
"Blitz: 843 ratings"
++ Do not play blitz or bullet. It breeds bad habits: playing too fast and superficial.
"1. Some chess.com lessons have emphasized generating plans. How should I generate a plan?" ++ Main plan: hang no pieces or pawns.
"2. I am relatively defensive and positional, and tend to play slow-developing counterattacks and endgames. Would my performance be improved if I become more attacking?"
++ No, your performance will probably drop as you will make even more tactical mistakes.
"3. About Openings. The recent drop in rapid rating is contributed by terrible play (for whatever reason) and openings."
++ No, you lose because you hang pieces and pawns, not lose because of the opening.
"How much should I know about openings?" ++ Just the opening principles are enough.
"For example, should I know the book opening of KIA, or I should learn the traps in KIA for white and black?" ++ None of these. Just play. develop pieces into play and control the center.
"It is really interesting to see most of my games have around 3 book moves while some lines can be over 10 book moves." ++ Do not worry about book moves.
"4. About Pawn Structures. Neglect the missing kings." ++ Neglect the essential...
"Which pawn structure looks better if it is in a.) middlegame and b.) endgame?"
++ That depends on the positions of the kings and the other pieces. With a black king on c3 black is better, with a white king on c6 white is better. With a white light square bishop black is better. With a black dark square bishop white is better. With a white knight at d5 white is better. With a black knight on d4 black is better.
Some statistics on myself:
Rapid: 1135 ratings (hi 1221), ~600games
Blitz: 843 ratings (hi 851), ~200games
And a few games in bullet, that I always timeout so I hate bullet...
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How can I improve?
1. Some chess.com lessons have emphasized generating plans. How should I generate a plan?
2. I am relatively defensive and positional, and tend to play slow-developing counterattacks and endgames. Would my performance be improved if I become more attacking?
3. About Openings. The recent drop in rapid rating is contributed by terrible play (for whatever reason) and openings. How much should I know about openings? For example, should I know the book opening of KIA, or I should learn the traps in KIA for white and black? It is really interesting to see most of my games have around 3 book moves while some lines can be over 10 book moves.
4. About Pawn Structures.
Neglect the missing kings. Which pawn structure looks better if it is in a.) middlegame and b.) endgame?
Thank you for your time in reading this post! I would be very grateful if you could drop down some words of wisdom :>