CLASSES OF CONDITIONAL PROTASES.
[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.
[2025.] The indicative in a conditional protasis may state present, past, or future time.
The mood and tense of the apodosis are determined by the sense. The following combinations occur: