One question new chess players often ask is when does the opening end and the middle game begin. There are no hard and fast rules for this and even strong players will disagree whether a specific position is in the opening or the middle game. It's not important to agree. The terms for the three phases of a game are for convenience. They are like historical ages. We may not agree on exactly which year the Bronze Age began, but it is a useful concept for talking about human history.
There is an extremely useful method for knowing when you are still in the opening. Count the developing moves. Some moves develop your game, some moves don't.
What is a developing move? It's a move that brings at least one of your pieces into action. For example, your Bishops are blocked by their own Pawns at the beginning of the game. To develop a Bishop you need to push at at least one Pawn. To develop the Kingside Bishop, you need to push the e- or g-Pawn. That's a developing move. Moving the Bishop itself is another developing move.
What aren't developing moves? Capturing or recapturing your opponent's pieces usually develop nothing, so these don't count as developing moves, even though they are essential.
The real value of counting developing moves is that it gives us an idea about who is leading in development. It also gives us a clue about what moves to consider next.
[See also the Initial Position.]