By the tenses (‘tense’ from
1. The time of an action: present, past, future.
2. The stage of an action: action continued or repeated (in process of development), action simply brought to pass (simple occurrence), action completed with a permanent result.
a. The time of an action is either absolute or relative. Time that is absolutely present, past, or future is reckoned from the time of the speaker or writer. Time that is relatively present, past, or future in dependent clauses is reckoned from the time of some verb in the same sentence. In dependent clauses Greek has no special forms to denote the temporal relation of one action to another (antecedent, coincident, subsequent), but leaves the reader to infer whether one action happened before, at the same time as, or after another action. The aorist is thus often used where English has the pluperfect ( cross1943). See cross1888, cross1944. Unless special reference is made to relative time, the expressions “kind of time,” “time of an action,” in this book are used of absolute time.
b. In independent clauses only the tenses of the indicative denote absolute time; in dependent clauses they express relative time. The tenses of the subjunctive, optative, imperative, infinitive and participle do not refer to the differences in kind of time. Thus γράφειν and γράψαι
c. Even in the indicative the actual time may be different from that which would seem to be denoted by the tense employed. Thus the speaker or writer may imagine the past as present, and use the present in setting forth an event that happened before his time ( cross1883); or may use the aorist or perfect of an event that has not yet occurred ( cross1934, cross1950).
d. In the subjunctive, optative (except in indirect discourse), and imperative the kind of time is implied only by the mood-forms, not by the tenses. The relation of the time of one action to the time of another usually has to be inferred in all the moods.
e. The stage of an action is expressed by all the tenses of all the different moods (including the participle and infinitive).
f. The action of the verb of a subordinate clause may overlap with that of the verb of the main clause. See cross2388.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].