ferreus essem, nisi te amarem, I should be hard-hearted unless I loved
    you; but—­

    ferreus essem, si te non amarem, I should be hard-hearted if I did
    NOT_ love you_.

In the first example, it is the notion of loving you that is negatived, in the second, the notion of loving.

2.  Si non (si minus) is regularly employed:—­

  a) When an apodosis with at, tamen, certe follows; as,—­

    dolorem si non potuero frangere, tamen occultabo, if I cannot crush my
    sorrow, yet I will hide it.

  b) When an affirmative protasis is repeated in negative form; as,—­

    si feceris, magnam habebo gratiam; si non feceris, ignoscam, 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,—­

    hoc si assecutus sum, gaudeo; si minus, me consolor, if I have
    attained this, I am glad; if not, I console myself.

3.  Sin.  Where one protasis is followed by another opposed in meaning, but affirmative in form, the second is introduced by sin; as,—­

hunc mihi timorem eripe; si verus est, ne opprimar, sin falsus, ut timere desinam, 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 (non, nemo, nihil); as,—­

    nihil cogitavit nisi caedem, he had no thought but murder.

  a.  Non and nisi are always separated in the best Latinity.

5.  Nisi forte, nisi vero, nisi si, unless perchance, unless indeed (often with ironical force), take the Indicative; as,—­

    nisi vero, quia perfecta res non est, non videtur punienda, unless
    indeed, because an act is not consummated, it does not seem to merit
    punishment.

Conditional Clauses of Comparison.

307. 1.  Conditional Clauses of Comparison are introduced by the particles, ac si, ut si, quasi, quam si, tamquam si, velut si, or simply by velut or tamquam.  They stand in the Subjunctive mood and regularly involve an ellipsis (see Sec. 374, 1), as indicated in the following examples:—­

    tantus patres metus cepit, velat si jam ad portas hostis esset, as
    great fear seized the senators as (would have seized them) if the enemy
    were already at the gates;

    sed quid ego his testibus utor quasi res dubia aut obscura sit, but
    why do I use these witnesses, as (I should do) if the matter were
    doubtful or obscure;

    serviam tibi tam quasi emeris me argento, I will serve you as though
    you had bought me for money.

2.  Note that in sentences of this kind the Latin observes the regular principles for the Sequence of Tenses.  Thus after principal tenses the Latin uses the Present and Perfect (as in the second and third examples), where the English uses the Past and Past Perfect.