Aorist: Indicative
Simple Past

What happened? (story-telling, actions, specific/precise/one-off facts.
And what happened next?
Who did what? when? where? why?

By contrast the Imperfect tells the surrounding circumstances or state: what was going on? used to be? happened repeatedly? in general?

The 1st aorist, or aorist in -a (a being the 1st letter of the alphabet) has an alpha in most endings when the 2nd aorist has ο/ε.

 ἐ   past tense marker 
 κέλευσ   aorist stem 
 ε(ν)  past tense ending, 3rd person singular 

 

 Past tense marker:   ἐ  added to the front of the verb (if the verb starts with a consonant)
 1st aorist stem:  κέλευσ often σ added to present tense stem, like for future) 
 Secondary  endings for past tenses as opposed to  primary  endings for present and future

 

Active Voice
Primary endings   Secondary Endings
Present    Future Imperfect    1st Aorist
orders will order was ordering ordered
 κελεύ   ω   κελεύσ   ω   ἐκέλευ   ον   ἐκέλευσ   α 
 κελεύ   εις   κελεύσ   εις   ἐκέλευ   ες   ἐκέλευσ   ας 
 κελεύ   ει   κελεύσ  ει   ἐκέλευ  ε (ν)  ἐκέλευσ   ε (ν)
 κελεύ   ομεν   κελεύσ   ομεν    ἐκελεύ   ομεν   ἐκελεύσ   αμεν 
 κελεύ   ετε   κελεύσ   ετε   ἐκελεύ   ετε   ἐκελεύσ   ατε 
 κελεύ   ουσιν   κελεύσ   ουσιν  ἐκέλευ   ον   ἐκέλευσ   αν 

The 2nd aorist indicative (=simple past tense) has the same secondary endings as the imperfect
The 1st aorist indicative (=simple past tense) also has secondary endings but with an α instead of ο/ε (except for the he, she, it form)