sin

Sin is si with the old negative particle -ne fused on, and it introduces the second branch of a two-part condition: "if this ... but if instead that". You will almost never meet it without a si clause standing in front of it, which is why it is worth learning as half of a pair rather than on its own.

Three tags do most of the work. Sin minus and sin aliter both mean "if not, otherwise", and are what a Roman writes where English says "failing that". Sin autem is the Vulgate's habit and turns up constantly in ecclesiastical prose. Grammatically it behaves exactly as si does, taking indicative or subjunctive according to the kind of condition; sin changes the logic of the sentence, not the mood of the verb.