1st 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: what was going on? used to be? happened repeatedly? in general?
 
Caption of the picture introducing Chapter 12a:
προσ-εχώρησεν ἀνήρ τις ἅμαξαν ἐλαύνων 
A man approached, driving a wagon.
 
 ἐ   past tense marker 
 χώρησ   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:  χώρησ often σ added to present tense stem, like for future) 
And, as happens in the future, for contract verbs
stem ending α, ε lengthen to η and
stem ending ο lengthens to ω.
 First Aorist Active Endings: 
 
Who? approached
 I
 you
 he/she/it  
 we
 you
 they
προσ-εχώρησ
προσ-εχώρησ
προσ-εχώρησ
προσ-εχωρήσ
προσ-εχωρήσ
προσ-εχώρησ
α
ας
ε(ν)
αμεν
ατε
αν

The 1st aorist (like the 2nd aorist and the imperfect) has secondary endings but with an α instead of ο/ε (except for the he, she, it form)

 Note 
As προσχωρέω "I approach" is a compound verb I inserted a hyphen to show that the past tense marker is put in front of the base verb, NOT before the compound prefix.
 

More examples for practice:

Picture chapter 11a: They arrived at their brother's house and then... ὁ Δικαιόπολις κοψε τὴν θύραν.

What happened next? Dikaiopolis knocked on the door.
So how would you ask who knocked?
How would you say that you personally knocked?
that the guards knocked?

Why ψ instead of ἔκοπτσα? Because in ancient Greek τ is always dropped before sigma. There is NO τσ combination, nor δσ, θσ. τ, δ and θ are always dropped when supposed to be followed by a sigma, in the future as well as in the 1st aorist.
Which leaves a ps sound that must be written ψ because there is no such thing as a πσ combination. Just as there is no κσ, γσ or χσ, all pronounced ks and spelt ξ.

 Note  The only time you see a κσ instead of ξ is in compound words like ἐκ-σώζω for instance, where the word beginning with a sigma is preceded by the preposition/prefix ἐκ.

 Examples 

 
Who?
Present Indicative  
does it
Future Indicative  
will do it
Aorist Indicative  
did it
I 
you (sg) 
he,she,it  
we 
you (pl) 
they 
σκάπτω
πέμπεις
γράφει
διώκομεν
παίζετε
πείθουσιν
σκάψω
πέμψεις
γράψει
διώξομεν
παίσετε
πείσουσιν
ἔσκαψα
ἔπεμψες
ἔγραψε
ἐδιώξαμεν
ἐπαίσατε
ἔπεισαν