The playing principle
World of Card Games describes following the led suit when able. A player unable to follow may play any card. The highest trump wins; without trumps, the highest card of the led suit wins.
Separate permission from outcome
In a practice, ask two different questions: “Which cards may I play?” and “Which played card wins?” Answering the second does not answer the first. Make the learner locate the relevant rule before deciding what to put down.
Slow down the sample
Leave all three played cards visible while you explain the result. Identify the led suit and the trump suit aloud. This is a teaching exercise, not a new rule or a guarantee that one particular card is strategically best.