Conditional Periods.
[2016.] A protasis introduced by sī, so, if, or nisi, unless, if not, states a condition; the apodosis states action occurring under that condition. The conditional protasis and apodosis combined make a Conditional Period.
Thus, sī diēs est, if it is day, is a conditional protasis; combined with an apodosis, lūcet, it is light, it makes a conditional period: sī diēs est, lūcet, Inv. 1, 86, if it is day, it is light.
[2017.] A parenthesis with ut ([1943]) is added when the speaker asserts that the action of the protasis is not only assumed, but actually occurs: as, sī virtūs digna est glōriātiōne, ut est, beātus esse poterit virtūte ūnā praeditus, Fin. 4, 51, if virtue is entitled to glorification, as it really is, he will find it possible to be happy in the possession of virtue alone. sī nox opportūna est ēruptiōnī, sīcut est, haec profectō noctis aptissima hōra est, L. 7, 35, 10, if night is always favourable for a sortie, and it always is, this particular hour of the night is surely the very best time.
[2018.] The apodosis is usually declarative. Often, however, it is interrogative, exclamatory, or imperative, or it may take any other form which the thought or the context may require. The apodosis has rarely a correlative to sī: as, igitur, it follows that, idcircō, for all that, tum, then, ita, sīc, only, eā condiciōne, on condition; at, but, tamen, nevertheless, certē, saltem, at any rate, tum dēnique, tum dēmum, then and not till then.
[2019.] sī is sometimes followed by quidem or, from Cicero on, by modo: sī quidem, that is if, since, even if, sī modo, if only. sī tamen, at least if, is found in Lucretius, Sallust, the Augustan poets and in late writers. sīve ... sīve (seu . . . seu) or, in old Latin, sī . . . sīve, whether . . . or, with the indicative or the subjunctive of the indefinite second person ([1556]), leaves a choice between two cases possible. By abbreviation of the protasis sīve becomes a coordinating particle: see [1672].
[2020.] The negative of sī is sī nōn, if not (sī nēmō, sī nūllus, &c.), or nisi, unless, if not, used especially of an exception or after a negative, nisi sī, chiefly in old, colloquial, or late Latin, or, particularly in solemn language or poetry, nī is sometimes used for nisi. A restriction, usually an ironical afterthought, may be introduced by nisi forte (rare before Cicero) or nisi vērō (in Cicero and Pliny the Younger) with the indicative.
nisi is sometimes found in an adversative sense in old and colloquial Latin, especially after nesciō; from Cicero on, it may be strengthened by tamen. For nisi quod, see [1848].
[2021.] When a second conditional period is opposed to a first, it is sometimes introduced by sī (or sī autem), but usually by sīn (or sīn autem). If the second period is negative, and its verb is not expressed, minus or aliter is preferred to nōn.