Learn Russian: Благодаря репетиции она выучила роль, до того как открыли занавес. - Thanks to the rehearsal she learned the role before they opened the curtain.

No, because до is a preposition and can only be followed by a noun or pronoun. Because the second part of the sentence has a working verb (открыли), it is a full clause. To connect a clause meaning 'before', you must use the full conjunction до того как.

Where does the comma go when using до того как?

Usually, the comma is placed right before the whole phrase to separate the main clause from the time clause: ...роль, до того как.... However, if a speaker wants to heavily emphasize the timing, you might occasionally see the comma placed right before как instead: ...до того, как....

Who is the subject of открыли (opened)?

There isn't a stated subject. In Russian, if the person doing the action is unknown or unimportant to the story, you can simply use the plural past-tense verb (открыли) and leave out the pronoun (они). This acts like the generic 'they' in English or the passive voice ('before the curtain was opened').

Why does репетиции end in -ии?