The action denoted by a verb may be defined by the tense of the verb
(a) As respects its progress. Thus it may be represented as in progress, or as completed, or indefinitely, i.e. as a simple event without reference to progress or completion.
(b) As respects its time, as past, present, or future.
The tenses of the Indicative mood in general define the action of the verb in both these respects.
The tenses of the other moods in general define the action of the verb only as respects its progress.
REMARK. The chief function of a Greek tense is thus not to denote time, but progress. This latter function belongs to the tense-forms of all the moods, the former to those of the Indicative only.