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がいつ戻ってくるか

when someone will come back

→ が
phrase Rare #39,651
when someone will come back Used in an indirect question to ask or state when someone will return. The person before が can change depending on the context.
JA: 友達がいつ戻ってくるか知っていますか。
EN: Do you know when your friend will come back?

彼がいつ戻ってくるかわかりません。

I don't know when he'll be back.

彼等がいつ戻ってくるか知っていますか。

Do you know when they'll be back?

トムがいつ戻ってくるか知ってる？

Do you know when Tom will be back?

彼女がいつ戻ってくるかは分かりません。

I don't know when she'll be back.