Learn Russian: Актёр стоял за кулисами, перед тем как выйти на сцену. - The actor stood backstage before going out onto the stage.

When the same person is performing both actions (the actor was standing, and the same actor goes out), we use the infinitive after перед тем как. If a different person were doing the second action (like "before the director arrived"), you would use the past tense instead of the infinitive.

Where does the comma go when using перед тем как?

The standard rule is to put the comma right before the entire phrase (перед тем как) to separate the main clause from the "before" clause. You might occasionally see native speakers put the comma before как depending on emphasis, but placing it before перед is the standard style taught to learners.

Why is за кулисами in the instrumental case?

The preposition за takes the instrumental case when it answers the question "where?" (static location). Кулисы is a plural-only noun that refers to the wings of a stage, so за кулисами literally means "behind the wings," which is how Russians say "backstage."

Why do we use the imperfective стоял but the perfective выйти?

Стоял (was standing) describes a continuous, ongoing background state, which requires the imperfective aspect. Выйти (to step out / to exit) is a single, completed event that happens after the standing, so it requires the perfective aspect.

Why is it на сцену instead of на сцене?

The verb выйти implies motion toward a destination (going onto the stage). When на shows direction or motion, it requires the accusative case (сцену). If the actor were already standing on the stage, you would use the prepositional case to show static location (на сцене).