proinde

Proinde is pro plus inde, and its core is "in like manner". Two jobs come out of that. Joined to an imperative or a jussive subjunctive it means "therefore, so then", and it is noticeably more urgent than igitur or itaque: a speaker uses proinde when he wants the audience to do something next. Paired with ac, atque, quasi or ac si it means "exactly as, just as if", and the ac si version takes the subjunctive in the tense a conditional would want.

The trap is perinde, which looks like a near-twin and is not. Perinde only does the comparative job, "in the same way as"; it never means therefore. Scansion is the other oddity: proinde is three syllables in prose, but poets regularly squeeze the oi into one and treat it as two.