Paragraph 5

ἔπειτα δὲ ἡ μήτηρ....

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ἔπειτα (adverb): and then, hereafter, thereafter, afterwards ...)
ἔπει δέ (conjunction): when someone does/did/will do something
ἐν ᾧ δέ while something is going on, while someone is doing something
ὁ πάππος πάντα λέγει, ἐπειτα δέ καθεύδει, κάμνει γάρ.
ἡ Μυρρίνη ἀκούει τὸν μῦθον, ἐπειτα δὲ καλεῖ τοὺς παῖδας καὶ λέγει αὐτοῖς ἄλλον καλὸν μῦθον.
ἐπεὶ δε πάρεισιν πάντες, καθίζουσιν καὶ δειπνοῦσιν.
ἐν ᾧ δὲ δειπνοῦσιν, λέγει τὸν μῦθον ἡ Μυρρίνη.
 

Difference between:

κάθιζε, ὦ Μέλιττα. Plain: sit down, Melitta!
σὺ δὲ, ὦ Μέλιττα, κάθιζε. Emphasis: And you, M, sit down.
σὺ δὲ, ὦ Μέλιττα, κάθιζε καὶ σύ. Stronger emphasis: And you Melitta, sit down you too.

ἐγὼ γὰρ μέλλω καλὸν μῦθον ὑμῖν λέγειν.

Why ἐγώ?
Difference between μέλλω γάρ (α) and
Difference between ἐγὼ γὰρ μέλλω (β)
(α) normal: I'm going to tell a story
(β) emphasis: after they have told their stories, it is my turn to tell a tale. So now it is me who's going to speak, not you, nor anyone else either.
If in oral English we put the stress on "I", in Greek we use the pronoun 'ἐγώ'
In oral English "as for you", in Greek 'σύ' 
I see you but you don't see me:
ἐγὼ μὲν σὲ ὁρῶ, σὺ δὲ οὐχ ὁρᾷς ἐμέ.
I provide you with food, you don't provide me with anything (οὐδέν)
ἐγὼ μὲν σοῖ σῖτον παρέχω, σὺ δὲ οὐ παρέχεις ἐμοῖ οὐδέν.
We use personal pronouns in English far more than in Greek. Because Greek verb-forms always tell who performs the actions. And only when we want to emphasise the subject do we use pronouns in the subject case.
 

So what did happen with those other pronouns in the above:

you see me: με ὁρᾷς
I see you: σε ὁρῶ
you give it to me: μοι παρέχεις
I give it to you: σοι παρέχω
 

In the singular (relating to me and to you, one you) we have two different forms of pronoun:

unaccented for neuter tone, plain me, you, to me, to you:
(gen.) μου - σου, (dat.) μοι - σοι, (acc.) με - σε
accented for emphasis:
(gen.) ἐμοῦ - σοῦ, (dat.) ἐμοῖ - σοῖ, (acc.) ἑμέ - σέ
 

Examples:

Come here, sit with me. No, not with him, with me.
ἔλθε δεῦρο, κάθιζε μετά μου. οὐ, οὐ μετ' αὐτοῦ, μετ' ἐμοῦ.
And then the savage dog attacks me.
ἔπειτα δὲ ὁ ἄγριος κύων μοι ἐμπίπτει.
And then the savage dog attacks me.
ἔπειτα δὲ ὁ ἄγριος κύων ἐμπίπτει ἐμοῖ.
Little Sophy runs after me all the time.
ἡ μικρὰ Σοφία αἰεὶ τρέχει μετά με.
Always after me, my friend, not after you.
αἰεὶ μετ' ἐμέ, οὐ μετὰ σέ, ὦ φίλε.