ni

Ni is the older, shorter form of nisi, "if not, unless". Classical prose mostly leaves it to verse, legal formulae and a handful of fixed phrases, above all ni fallor, "unless I am mistaken". It behaves exactly as nisi does: indicative for an open condition, subjunctive for the unreal ones, with the sequence of tenses working as usual.

Two look-alikes to keep off it. Ne is a different word, "lest, that not", and it introduces purpose and fear clauses with the subjunctive. The enclitic -ne, tacked on the end of the first word, marks a question and has nothing to do with either. Ni also survives in the wager formula ni ita est, "if that is not so", and in nimirum, "no wonder".