The one winning trick in rook and knight versus rook: the defending king is in the corner, the attacking king is adjacent, and the knight shields against the checks while the rook delivers mate on the back rank.
Rook and knight versus rook is drawn in general but not always. The losing configuration for the defender is a king in the corner with the attacking king right next to it, because then the knight has time to block the checks and the rook mates.