2. Sī nōn (sī minus) is regularly employed:—
a) When an apodosis with at, tamen, certē follows; as,—
dolōrem sī nōn potuerō frangere, tamen occultābō, if I cannot crush my sorrow, yet I will hide it.
b) When an affirmative protasis is repeated in negative form; as,—
sī fēceris, magnam habēbō grātiam; sī nōn fēceris, ignōscam, if you do it, I shall be deeply grateful; if you do not do it, I shall pardon you.
a. But if the verb is omitted in the repetition, only si minus or sin minus is admissible; as,—
hōc sī assecūtus sum, gaudeō; sī minus, mē cōnsōlor, if I have attained this, I am glad; if not, I console myself.
3. Sīn. Where one protasis is followed by another opposed in meaning, but affirmative in form, the second is introduced by sīn; as,—
hunc mihi timōrem ēripe; sī vērus est, nē opprimar, sīn falsus, ut timēre dēsinam, relieve me of this fear; if it is well founded, that I may not be destroyed; but if it is groundless, that I may cease to fear.
4. Nisi has a fondness for combining with negatives (nōn, nēmō, nihil); as,—