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#.