Opening

The opening is typically the first 10-15 moves of the game. The key objectives in the opening is to develop pieces, take control over the center of the board, and castle your king to safety. Most of the time pawns are developed first with the minor pieces being developed after. It is recommended to not move the minor pieces more than once in the opening or developing the queen too early. The opening theory of chess has gotten extremely developed with different 'openings' being created. Openings describe specific sequences of moves and prepare the player with the best responses to different variations that the opponent plays.

Middlegame

The middlegame starts when both kings are castled and pieces have been developed. The key objective in the middlegame is to start an attack on the enemy king. This is done through pushing pawns, creating 'open files' which allow pieces such as rooks to have control towards the king's defense, and using tactics (in particular, removing defenders of the king are great).

Endgame

The endgame happens when there are few pieces remaining in the game. Winning games from the endgame usually require pawn promotions or checkmating the king. There are guides on how to win specific endgames such as checkmating with a rook and king against the last standing enemy king.