Dialogue drôle dans un cours d’art mal choisi

What does “se tromper de” mean in French?

On its own, se tromper means “to be wrong” or “to make a mistake.” Add de plus a noun and it becomes “to get the wrong one.” So se tromper de bus is “to take the wrong bus,” and se tromper de classe is “to walk into the wrong class.” No article goes between de and the noun — drop “le” or “la” and put the noun straight after.

At the end of this art-class sketch, the confused student finally says “Je pense que je me trompe de classe” — “I think I’m in the wrong class.” That one line is the joke and a very useful pattern. The shape is always se tromper de + (bare noun): se tromper de jour (get the wrong day), se tromper de numéro (wrong number — “vous vous êtes trompé de numéro” is what you say on the phone), se tromper de route (take the wrong road), se tromper de chemin (go the wrong way). Two quick notes. The verb is reflexive, so you need me / te / se: je me trompe, tu te trompes, il se trompe. And without de, plain se tromper just means “to be mistaken”: tu te trompes = “you are wrong.”