“If that's the case” in Japanese
Short answer
If that's the case in Japanese is “そんなら” (sonnara). Full meaning: if that's the case ..., if so ..., that being the case ....
Other ways to say “If that's the case” in Japanese
やったら
yattara if that's the case
其れなら
sorenara if that's the case ..., if so ..., that being the case ...
それはそれで
sorehasorede in that case, (might not be what was expected or hoped for but) if that's the case
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