Learn French: Vous devriez fermer le robinet avant de partir. - You should close the tap before leaving.

When you want to say "before doing something" in French, you must use the phrase avant de followed by the infinitive of the verb. You cannot skip the de.

Can I use avant de if someone else is leaving?

No, you can only use avant de + infinitive when the person doing both actions is the same (here, "you" are closing the tap, and "you" are leaving). If the subjects are different, you have to use avant que followed by a conjugated verb in the subjunctive mood, like avant qu'il parte (before he leaves).

Why is the verb devoir in the form devriez here?

Devriez is the conditional form of the verb devoir. While the present tense vous devez means "you must" or "you have to", using the conditional vous devriez softens it to "you should", making it a piece of advice rather than a strict command.

Could I use sortir or quitter instead of partir in this sentence?