When do I use which?

In Ancient Greek the choice between present and aorist imperative was altogether subjective. The speaker could use a present imperative not only when from an objective point of view the situation called for a certain action, but also when only in the speaker’s opinion the situation necessitated an action.—The Greek Imperative, page 71

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Mind you, he means Classical when he says Ancient. Things change in Hellenistic/Koine; stay tuned...
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