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if that's the case ...; if so ...; that being the case ...
それなら明日の会議は延期しましょう。
If that's the case, let's postpone tomorrow's meeting.
用事があるのなら、それなら私が代わりに行きます。
In that case, I'll go instead.
時間がないの?それなら手伝うよ。
You don't have time? Then I'll help you.