Any simple or unreal condition of present or past time, or any future condition, may refer to a customary or frequently repeated act or to a general truth. But for the present and past only (when nothing is implied as to fulfilment) there are two forms of expression: either a special kind of conditional sentence or (less frequently) the simple condition, as regularly in English and in Latin:
Present. Protasis: ἐά_ν (= ἐά_ν ποτε) with the subjunctive; apodosis: the present indicative ( cross2337).
Past. Protasis: εἰ with the optative; apodosis: the imperfect indicative ( cross2340).
a. By reason of the past apodosis, the optative in the protasis refers to the past. Only in this use (and when the optative in indirect discourse represents a past indicative) does the optative refer distinctly to the past.
b. The present subjunctive and optative view the action as continuing (not completed); the aorist subjunctive and optative, as simply occurring (completed). The tenses of the protasis have no time of themselves, but usually the action of the present is relatively contemporaneous with, the action of the aorist relatively antecedent to, the action of the main verb.
c. The indicative forms in the protasis are more common in temporal and relative sentences. Observe that it is the character of the apodosis alone which distinguishes the special kind of general condition from the two forms of future conditions.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].