Openings while studying games

How should I treat the opening while studying a chess game? I'm using the method of guessing the moves of the player who won the game, but it's not worth my time trying to guess opening theory. When do I know the opening is over in master games? (I am studying Tal-Botvinnik).

@1

"How should I treat the opening while studying a chess game?" ++ Do not worry about openings. You do not win or lose because of the opening, but because of tactical mistakes.

"I'm using the method of guessing the moves of the player who won the game"
++ That is good practice, but do so for the middle game and the endgame.

"it's not worth my time trying to guess opening theory"
++ No, it is not. But for the middle game and endgame it is good.

"When do I know the opening is over in master games?"
++ You can look up in a data base if there is a predecessor game with the same position.

"I am studying Tal-Botvinnik" ++ Excellent. Especially the 1st match. In the 2nd match Tal was ill.

Instead of trying to memorize specific moves or variations, try to understand the opening principles behind each move. Pay attention to concepts such as controlling the center, developing pieces and preparing for king safety.

@4

"How do i know if the opening is over and both players started to think instead of playing theory?" ++ By checking a data base.