2022. Conditional protases may be divided into two classes:
2023. I. INDETERMINATE protases, that is such as merely suppose an
action, without implying either its occurrence or its non-occurrence;
these may take:
(A.) Any tense of the indicative required by the sense; or (B.) the
present subjunctive, less frequently the perfect subjunctive, to express
a condition in the future.
2024. II. Protases of ACTION NON-OCCURRENT, that is such as suppose
action not taking place. These take the imperfect or pluperfect
subjunctive.
Thus, in the period #sī diēs est, lūcet#, _Inv._ 1, 86, _if it is day,
it is light_, the protasis _if it is day_ is indeterminate, neither
implying that _it is_, or _is not day_. But in #sī vīveret, verba eius
audīrētis#, _if he were alive, you would hear his evidence_, _RC._ 42,
the protasis denotes action non-occurrent, _if he were alive_, implying
_but he is not_. The whole period, like the protasis, is either an
_Indeterminate Period_ or a _Period of Action non-occurrent_.