“unless” in Latin

nisi: ni: si non (nisi is used when the whole conditional clause is negatived, without particular stress on anyone notion contained in it: si non is used when there is emphasis on one notion; i. e., the verb, which is contrasted with the opposite affirmative proposition expressed or understood. But even where there is opposition, nisi is sometimes used; i. e., the opposition is not marked; e.g., matris verbis Philocomasium arcessito, ut si itura siet Athenas, tecum eat: nisi eat, te soluturum esse navim. Plaut., Mil. Glor., 4, 4, 48; Haase ad Reisig., p. 457). Unless perchance, nisi forte: nisi vero (often ironical: with indicative; hence not nisi forte aptius et commodius esset scribere, etc., but est). In Cic., nisi vero [ = eἰ μή γε] often introduces a supposition, the manifest absurdity of which is to establish the truth of the other supposition, previously stated to be true (Hand, iv., p. 233). ☞ An excepted case (added correctively) is more emphatically stated by nisi si; e.g., noli putare me ad quenquam longiores epistolas scribere, quam adte, nisi si qui ad me plura scripsit, etc. So nisi si quis est, etc. (Cic.): ☞ praeterquam si is post-classical; e.g., Plin., 8, 25, 39.