A knight struggles to stop an advancing pawn: it covers long distances slowly. Its job is to control the squares in front of the pawn, above all the promotion square.
The black pawn on c3 will promote in two moves. Get there in time with your knight.
The hardest case is against a rook pawn: in the corner the knight has little room. When defending with a knight, look at the squares in front of the pawn first, then count how many moves the knight needs to get there.