chess phases

It is quite hard to decide when each phase ends and which begins, as it really depends on the game itself. It also isn't too important to distinguish each phase from each other because the game flows into each phase pretty seamlessly. However, you could say that the opening ends after full development, and middlegame ends after you have about 3 or less pieces on the board for both sides. It is incredibly subjective though, you'll see some grandmasters talk about the opening 30 moves deep (see Marshall Ruy Lopez), and some of them say that the endgame can start as early as move 10 or so (see Berlin Ruy Lopez). The only reason why this distinction would be important at our level is if you're trying to determine weaknesses in your play (e.g., not recognising common endgame patterns like the Lucena position, or not knowing what to do on move 3 in the opening). Otherwise, don't worry too much about the distinction. Calculation and basic strategic understanding apply to all phases of the game and they'll take you a long way!

"how when reviewing a game, you can tell what phase of the game you're in?"
When reviewing a game, look in a data base up to which point you were following a previously played game, the so called predecessor game. Up to that point = opening. From that point = middle game.
Endgame is related to the number of pieces after trades. Once there are 2 or more less pieces per side, not counting pawns nor the omnipresent kings, it is an endgame.

good morning, and thank you for your replies! interesting to read, and has sparked other questions related to chess phases, play and review.

@tygxc, this seems to be how a (computer) chess program would determine what phase it is in? if that's the case, how interesting to know that this is how they think. even if it was programmed by the "creator" to think about the phases like this, still that's interesting! bc similar to what @quicksmurfer said, " middlegame ends after you have about 3 or less pieces on the board for both sides." however, what she said lines more with what i have been noticing. and it is the subjectiveness of chess that I'm interested in understanding.

" some grandmasters talk about the opening 30 moves deep"  or  "you could say that the opening ends after full development" and this is what i find interesting about chess! and what i would like to understand is how a player thinks. and to know and understand about each phase is crucial to chess play. the beginning and ending are openly studied, because they are "easier" to repeat, where as in the middle game, players are left to rely on skill, luck (by blunder of their opponent) or their ability to recall pervious openings and ends in an attempt to understand their middle and "move on to the end they know."

with this way of understanding chess phases, i can see how subjective chess can be. and its amazingly indescribable how alive chess can feel. and it is because of this feeling, that i think i am so interested in chess. and so, before i continue on with anything else, ill pause here. I hope this continues, and that my questions and thoughts are understandable? im looking forward to reading your thoughts! and thank you again to all who participate! it means a lot

take care, and best of luck in your games!!