What changed, and candidate moves is lesson 2 of 2 in How to choose a move, a strategy course suited to players rated around 800 (beginners). Your opponent's last move is a message. Read it first. Then compare a few moves before you choose. It has 2 interactive exercises: 2 find-the-move puzzles. You play every move on a board and get feedback straight away. The lesson is free.
What this lesson explains
Black just played Ng4. Ask what it changed. Does it attack something? Does it open a line? Did it leave a piece without a guard? Here the knight and the bishop on c5 both aim at f2, and only your king guards it.
Recap.
First, ask what your opponent's last move changed.
Then list every check, capture and threat, for both sides.
Pick two or three candidate moves and play each one in your head.
Choose the move whose worst answer still leaves you fine.
What you practise
Black threatens Nxf2, which would fork your queen and your rook. Deal with the threat.
Compare three candidates: Nxd5, e3 and Nf3. The pawn on d5 looks free, because the knight on f6 seems pinned to the queen. Pinned means it cannot move without exposing a more valuable piece. Play each candidate in your head, then choose.
Worked examples
Black threatens Nxf2, which would fork your queen and your rook. Deal with the threat. Good. f2 is guarded twice now, and the knight jump does not work.
Compare three candidates: Nxd5, e3 and Nf3. The pawn on d5 looks free, because the knight on f6 seems pinned to the queen. Pinned means it cannot move without exposing a more valuable piece. Play each candidate in your head, then choose. Good. You did not fall into the trap. If Nxd5, then Nxd5, and if your bishop takes the queen, Bb4 check forces you to give your queen back. Black ends up a piece ahead.
chessGM