Learn French: Le gâteau doit refroidir avant que nous le mangions. - The cake must cool down before we eat it.

The phrase avant que (before) is a time conjunction that always requires the subjunctive mood. Because of this rule, we must use the subjunctive form mangions rather than the regular present tense mangeons.

Wait, doesn't mangions look exactly like the past (imperfect) tense?

Yes, it does! For regular -er verbs like manger, the nous and vous forms in the subjunctive mood are spelled identically to their imperfect tense forms. You can tell it is the subjunctive here because it directly follows the trigger avant que.

Could I simplify this and just say avant de le manger?

Not in this sentence. You can only use avant de followed by an infinitive if the subject doing both actions is the same person or thing. Here, the cake is the subject doing the cooling, but we are the subject doing the eating. When the subjects change, you must use avant que followed by a conjugated verb in the subjunctive.

Why does le go before the verb in the second half of the sentence?

The word le is a direct object pronoun standing in for the cake (it). In French, object pronouns are placed immediately before the specific verb they receive the action of, which in this case is mangions.