How does aspect work in Greek?

Aspect in Greek refers to whether the action is continuous or completed. For example, the aorist 'ἐλυσα' I loosened, suggests that I have loosened it once and the action has been completed. Compare this to the imperfect 'ἐλυον' I was loosening suggests that it is a continuous action - I was at that time still loosening or were in the process of loosening.