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This is a very basic and general question (and my first on this website), so feel free to tell me if this is inappropriate.
Besides specific strategies and opening sequences, there are some fairly general things about openings that are quite intuitive and, on the whole, not too difficult to follow: try to get out as many pieces as soon as possible and to position them on their most effective squares, try to get control of the center, try to castle when appropriate, put pressure on this square, do not move that pawn and so on. But when all the pieces are out, well placed, and I have castled, I have no idea about what I should do next. It might sound silly, but I feel that everything would compromise the balance I have just created. Are there any general pieces of advice, any general principles that one should try to apply? Or just any standard lines of thought that one could roughly follow? I feel like there is a huge gap between opening and endgame that I have no idea how to fill, and that I usually start out well in the opening and end being in disadvantage in the endgame because I did some silly moves in the middlegame because I did not know what to do.
For instance: I was playing a match against a computer opponent. I managed to get out all my pieces (horses on c3 and f3, pawns on d4 and e4, bishops on the sides, putting pressure on the f7 square and so on, all very classical) and to castle and the opponent had only been moving his horses around aimlessly by that time! He had not even moved a pawn! A good player would be sure to win, starting from such a situation. But in my case, whatever I tried turned out to be a bad idea and the counter actions of the opponent where always effective and in the end he won.
I should mention that I am reading "Logical chess move by move" by Irving Chernev. I haven't got vey far yet, but even the few pages I've read were very illuminating as far as positional play in the opening is concerned. But somehow I did not manage to take away any general advice on how to proceed in the middlegame: as I said, once everything is well placed, whatever I do seems to destroy its balance...