Learn Portuguese (Portugal): O maestro olha para as flautas antes de elas começarem a tocar. - The conductor looks at the flutes before they start playing.

Because it follows the preposition de (in the phrase antes de) and has a specific subject, elas (the flutes). When an infinitive has its own subject and follows a preposition, Portuguese uses the Personal Infinitive. Since elas is plural, we add the plural ending -em to the basic infinitive começar.

Why is there an a between começarem and tocar?

In Portuguese, the verb começar (to start or begin) requires the preposition a when it is followed by another action. So, to say start playing, you must use começar a tocar.

Why do we use para after olha? Doesn't para usually mean for?

While para often means for, the verb olhar (to look) pairs with the preposition para to mean looking at a specific target. So olha para as flautas translates to looks at the flutes.

Why is the subject elas explicitly stated before começarem?

While Portuguese often drops subject pronouns, including elas here makes it perfectly clear that the flutes are the ones starting to play, not the conductor (o maestro). It safely separates the subject of the first action from the subject of the second action.