Learn Russian: Режиссёр пошёл за кулисы, до того как открыли занавес. - The director went backstage before they opened the curtain.

до is a preposition that can only be followed by a noun or pronoun (like до спектакля - before the play). Because the second part of this sentence contains a verb (открыли), you must use the conjunction phrase до того как (or перед тем как) to connect the clauses.

Why is the verb открыли in the plural if no specific people are mentioned?

Russian frequently uses a third-person plural verb without the subject pronoun они to describe an action performed by unnamed or irrelevant people. In English, this often sounds best translated as a passive sentence: "before the curtain was opened."

Why is за кулисы in the accusative case? Doesn't за take the instrumental case?

The preposition за takes the instrumental case when it describes a fixed location (answering "where?"), as in Он стоит за кулисами (He is standing backstage). However, because the verb пошёл indicates motion toward a destination (answering "where to?"), за requires the accusative case here: за кулисы.

Can I use режиссёр to talk about the director or manager of a company?