307. 1. Conditional Clauses of Comparison are introduced by the particles,
ac sī,
ut sī,
quasi,
quam sī,
tamquam sī,
velut sī, or simply by
velut or
tamquam. They stand in the Subjunctive mood and regularly involve an ellipsis (see
§ 374, 1), as indicated in the following examples:—
tantus patrēs metus cēpit, velut sī jam ad portās hostis esset, as great fear seized the senators as (would have seized them) if the enemy were already at the gates;
sed quid ego hīs testibus ūtor quasi rēs dubia aut obscūra sit, but why do I use these witnesses, as (I should do) if the matter were doubtful or obscure;
serviam tibi tam quasi ēmerīs mē argentō, 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.