Learn Portuguese (Portugal): O maestro chega antes de vocês afinarem os violinos. - The conductor arrives before you tune the violins.

The phrase antes de (before) ends with a preposition (de). In Portuguese, verbs that follow a preposition usually need to be in the infinitive form, not the present tense. However, because we need to show exactly who is doing the tuning (vocês), we use the Personal Infinitive. We take the base verb afinar and add the plural ending -em.

Since the regular infinitive is afinar, could I just say antes de vocês afinar?

You could, but it sounds much less natural. In European Portuguese, whenever the person doing the action changes mid-sentence (here, it changes from the conductor to you all), it is strongly preferred to use the Personal Infinitive (afinarem). This helps make the new subject perfectly clear.

Could I drop the word vocês completely from this sentence?

Yes! Because the verb is conjugated as afinarem, the -em ending already tells us the subject is a plural group (they or you all). If it is already clear from the conversation who is being spoken to, saying antes de afinarem os violinos is perfectly natural and common.

I see antes de means "before". Do I always need to include the de?