When they arrived [simple fact] at his brother's house, Dikaiopolis knocked [simple fact] on the door.
[This lesson is supposed to be about 2nd aorist - irregular stem, same secondary endings as imperfect - but ἔκοψε is 1st aorist]
Nor does ἀφίκοντο simplify matters. Why not? Because the verb
to arrive is [ἀπο-ἱκνέομαι] shortened to ἀφικνοῦμαι
Where does the φ come from?
After dropping the ο of ἀπό in front of the ἱ we're left with p + aspirated sound which give us ph = φ in Greek letters.
On the same model shorten the following:
ἀπὸ ἵππων μάχονται 
ἑπὶ  ἵπποις πορεύονται 
to take away from
ἀπο-ἡγοῦμαι I lead away
Check here.
Back to ἐπεὶ δ' ἀφίκοντο,when they (had) arrived
Where is the past tense marker? It is a lengthening of the initial vowel ι to
ἁφ ικνοῦνται they arrive, present tense
ἀφ ῑκνοῦντο  they were arriving, imperfect tense
ἀφ ῑκ οντο, they arrived, full stop. AORIST tense
Full conjugation here