"In that case" / "if so":
* **** (colloquial)
* **** (colloquial)
* **** (politely colloquial)
* **** (formal, bookish)
Much like or which you may know, the conjugated forms of the copula can function as independent conjunctions (not conjunctive particles) at the beginning of a sentence, referring to the context before. It may sound surprising, but this is perfectly grammatical. The politeness or other nuances all conform to normal grammar that you need to adjust to.
>