Learn French: Tu prépares le moule avant que le gâteau aille au four. - You prepare the mold before the cake goes into the oven.

The phrase avant que (before) is a conjunction that always triggers the subjunctive mood. Because the action of the cake going into the oven hasn't happened yet, French requires the subjunctive aille rather than the indicative va.

aille looks completely different from aller. Is it an irregular subjunctive?

Yes, aller is highly irregular in the subjunctive. It uses the stem aill- for the singular pronouns and the third-person plural (que j'aille, que tu ailles, qu'il aille, qu'ils aillent). However, for nous and vous, it switches back to an all- stem (que nous allions, que vous alliez).

Could I say avant de instead of avant que in this sentence?

Not here, because there are two different subjects. You use avant de plus an infinitive when the same person does both actions. Since you (tu) prepare the mold but the cake (le gâteau) goes in the oven, the subject changes. When the subject changes, you must use avant que plus the subjunctive.

Why does the French say au four instead of for "into the oven"?