Aorist means the simple past. “I did something.”
Imperfect means an ongoing activity in the past. “I used to do something.” The word ‘imperfect’ even suggests ‘incomplete,’ and that’s not just a coincidence. The whole idea is ongoing, incomplete stuff.
Here are some examples. “James was sick.” (IMPF) vs. “James threw up.” (AOR) I don’t mean to imply that they are synonyms or two sides of the same coin. Not at all. Here’s another one: “We were looking for Johanna’s earring.” (IMPF) vs. “We found the earring.” (AOR)
Or, “We lived at 123 Main Street.” vs. “We bought a house at 123 Main Street.”
I was doing some exercises in Mastronarde’s book, and one was to translate English into Greek: “on that day the cavalry guarded the camp.” I racked my brain, and struggled, and tried to figure out if it should be translated in Greek as aorist or imperfect. I guessed aorist. … And was wrong. Grrr! … But, okay, “guarding” could fit better into an ongoing activity for a day, rather than an instantaneous action (AOR).
The next exercise was “they begged the king.” And I thought, “Okay, a one-time event … they went and begged the king. Obviously Aorist.” NOPE!!! Imperfect. Grrrr. I guess the sense is “they were begging the king.”
Mastronarde’s exercises are clearly designed 1) to cover material that has been presented, but 2) in a way that makes you think, rather than in the simplest possible fashion. It’s not a “trick question” at all. But I can see (at other times too) Mastronarde tries to use exercises that really force you to think about what’s been presented, rather than “oh .. of course.”