が戻ってくるまで

until ... comes back

→ が
phrase Rare #24,224
until ... comes back Used to describe something that continues up to the time when someone returns. The subject before が can change depending on context.
JA: 母が戻ってくるまで、ここで待っていて。
EN: Wait here until Mom comes back.

あなたが戻ってくるまでここにちゃんといます

I'll stay right here until you come back.

が戻ってくるまで一緒にいてあげて

Stay with them until I get back.

が戻ってくるまでここにいてほしいんだ

I want you to stay here until I come back.

あいつが戻ってくるまで待ってろ

Wait here until that guy comes back.