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?

 

Formation (Morphology)

Caption of the picture introducing Chapter 11a:

ἐπεὶ ἀφίκοντο εἰς τὴν τοῦ ἀδελφοῦ οἰκίαν, ὁ Δικαιόπολις ἔκοψε τὴν θύραν.
2 simple, specific actions: 1. They arrived (ἀφίκοντο: 2nd aorist middle)
2 simple, specific actions: 2. Dikaiopolis knocked on the door. (κοψε: 1st aorist)

How do we distinguish 1st aorist from 2nd? - 2 big differences:
  1. 2nd aorist has an important, mostly unpredictable stem change, see here.
    1st aorist is mostly regular

    In the supplementary reading ὁ Δημοκήδης τὸν βασιλέα ἰατρεύει there were 4 more 1st aorist forms:
    καὶ τὸν Δημοκήδη εἰς Σοῦσα κόμισαν
    ὁ βασιλεὺς ἀπὸ τοῦ ἵππου πεσὼν τὸν πόδα βλαψεν. (βλάπτω: I hurt, damage)
    τοὺς θεράποντας κέλευσε τὸν Δημοκήδη πάρ' αὐτὸν ἀγαγεῖν.
    ὁ βασιλεὺς ἰδὼν αὐτὸν θαύμασε.

     

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