To tell the story of a chess game, from a bird's eye view, is to tell a story of piece activity. From piece activity you can derive all manner of short term and long term strategic considerations.
I describe it with 4 levels of piece activity. Note you don't necessarily go from 1 to 2 to 3 to 4. You may go from 1 directly to 4 and then back to 3 etc during the course of a game. The hierarchy is only meant to show which is most favorable. (Just to be clear, pieces are non-pawns. Usually just the knights, bishops, rooks, and queen).
1) An inactive or badly placed piece. An easy example is any piece in the starting position. It has few or no squares it can move to, friendly pieces are blocking it in, and it is far away from threatening any of the opponent's pieces.
2) A mobile piece. Mobility means the piece has the option of moving to many different squares. It's useful because you can think of increasing mobility as increasing the probability the piece will do something concrete later (like tactics or checkmate). In the opening you do this by bringing pieces towards the middle of the board, or by putting bishops on long diagonals, rooks on open files, etc.
3) Infiltration. A highly mobile piece becomes even better after it uses this mobility to find a good post. A good post can be defensive in nature, but I use the word infiltration because I'm telling the story from the point of view of the winning side... so often a good post will be in the opponent's territory. This disrupts the opponent's lines of communication and starts to let you build up threats.
4) Finally, in the last stage, the piece comes into contact with real weaknesses. Often the most effective weaknesses to attack are the slowest... the pawns and/or the king. When you start making real threats you're more in control of exchanging one type of advantage for another (like space, material, and time). This is where you can jiu jitsu your way to a decisive advantage or outright win. This process usually culminates with a tactical sequence.
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Having said that, it's good to remember that during a real game you're almost never purely maneuvering pieces. Short term tactics mix with long term considerations. A strong player's moves will make sense in both worlds simultaneously.
I'm a bad player... I do one-two-three blunder in every game but I wanna ask not about tactics but about strategy of Chess game.
Anatoly Karpov explained that in the begging of Chess game you should develop your pieces.
In the end of the game... as I got it myself... you should try to make your pawn - Queen... or to eat your opponent's pawns.
But I don't know what you should do in the middle of the Chess game ?
Should I try to checkmate my opponent's King ?
Or should I push my pawns and minor pieces forward ?
Should I do something else ?
What is my main task ? And what is the best strategy for the middle of the game?
Help me, please.