- indicative in both halves: a plain condition, "if X is, Y is". A future or future perfect in the si clause is translated with an English present: si … peregeris "if you perform".
- present or perfect subjunctive: an imagined future, "if X were to happen, Y would follow".
- imperfect subjunctive: unreal now, "if X were so, Y would be"; pluperfect subjunctive: unreal past, "if X had been, Y would have been". The two can be mixed: siquid malum existeret, … prospexissent "if anything were evil, they would have provided for it".
After si, nisi, ne and num, quis and quid mean "anyone, anything": siquis, siquid. sin means "but if". At the start of a clause, quod si and quod nisi mean "and if", "and unless", with quod pointing back to what went before.