In general, tense in Greek involves two elements: aspect (kind of action, [sometimes call Aktionsart, though a difference does need to be made between the two]) and time. Aspect is the primary value of tense in Greek and time is secondary, if involved at all. In other words, tense is that feature of the verb that indicates the speaker's presentation of the verbal action (or state) with reference to its aspect and, under certain conditions, its time. [See Wallace's discussion of the significance of tense, aspect, and time, and also his discussion of the difference between portrayal and reality.]
The aorist tense "presents an occurence in summary, viewed as a whole from the outside, without regard for the internal make-up of the occurrence" (Fanning, 97). It may be helpful to think of the aorist as taking a snapshot of the action while the imperfect (like the present) takes a motion picture, portraying the action as it unfolds. In the indicative, the aorist usually indicates past time with reference to the time of speaking (thus, "absolute time"). Aorist participles usually suggest antecedent time to that of the main verb (i.e., past time is a relative sense).
Proleptic (Futuristic) Aorist
The aorist indicative can be used to describe an event that is not yet past as though it were already completed in order to stress the certainty of the event.
Rom 8:30 whom he justified, these he also glorified.
The Boston Christian Bible Study Resources:
http://www.bcbsr.com/greek/gtense.html
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