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. This corresponds to what is known as aspect in Greek grammar: the viewpoint or manner in which an action is portrayed.
(b) As respects its time, as past, present, or future. This is known as temporal reference, and it is most strictly observed only in the Indicative mood.
The tenses of the Indicative mood in general define the action of the verb in both these respects: aspect and time.… Learn Koine Greek