Hello,
The Opening stage is the starting stage of the game. It starts from move 1 to move 15-20 depending on the opening.
There are many openings chess players play. Positional players normally play 1.d4 - The queens pawn opening and attacking players play 1.e4 with white pieces.
Some openings with 1.e4 are :
Sicillian Defense - 1...c5
Caro-Kann - 1...c6
Alekhine - 1...Nf6
Pirc defense - 1...d6
Centre counter - 1...d5
And there are many openings with black's reply 1...e5, They continue with 2.Nf3 Nc6
Ruy Lopez - 3.Bb5 a6
.
Guicoo Piano - 3.Bc4 Nf6
Scotch - 3.d4 exd4 4.Nxd4
and black may also play 2...Nf6 which is called Petroff opening.
And similarly, openings with 1.d4 includes black's reply 1..d5, 1...f5, and 1...Nf6.
The move 1...f5 is called the dutch defense.
For 1..d5, there are openings like
2.c4 dxc4 - Queens Gambit Accepted
2.c4 e6 - Queens Gambit Declined
2.c4 c6 - The Slav defense
And the move 1...Nf6, white plays 2.c4 for which black replies
2...e6 3.Nc3 Bb4 - The Nimzo Indian
2...g6 3.Nc3 Bg7 - The Kings Indian defense.
I hope you find this instructive!
Thank you!