回答

(1) It's about (time) we (prepared) for the party.
※it is (about) time+S+V~ed(過去形) S~する時間・頃です
(2) (Without) you, I (wouldn't) (study) Japanese.
(3) (If) it (had) (not) (been) (for) Jack's home run, we would have lost the game.
※if it had not been for … もし…がなかったら[仮定法過去完了]/if it were not for … もし…がなければ[仮定法過去]

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