As always, what explains the difference between various forms is speech: ease of pronunciation, fluency. The way people speak is analysed and theories are elaborated afterwards to try and explain in abstract terms how the language works in practice, in real life. When learning a foreign language we've got to use our ears, train them to tell us: this is how it sounds right in ..... (Greek, French, Chinese....). If we want to learn a foreign language, we've got to train both our brain and our ears, pay attention to what they say and how they say it, the best possible way for them. Which quite often is not the way we would say it.
 
οὐ before consonants:
οὐ χαίρει
οὐ πολὺν σῖτον παρέχει.

οὐχ before aspirated vowels and diphthongs
οὐχ Ὁμηρος ἐστιν He is not (or: no) Homer.
οὐχ ἕκατον not a hundred
hectare, hectogramme,
hecatomb (originally sacrifice of 100 oxen: ἑκατὸν βόων)

οὐκ before all other, unaspirated vowels or diphthongs
οὐκ ἐν ταῖς  Ἀθήναις
οὐκ ἰσχυρός not strong
but: οὐχ ἱκανῶς (ἰσχυρός, ...) not (strong, ...) enough
 
 
 
 
 
 
 
 
 
 
 
 
 
 
αὐτός means
  • the same if placed after the article (if there is one) and before the noun
    ὁ αὐτός πόνος οὐ φέρει τὸν αὐτὸν μισθόν.
    or, poetically dropping the article:
    ὁ αὐτὸς πόνος οὐκ αὐτὸν φέρει μισθόν
    the same work doesn't [automatically] bring with it the same salary, pay, reward
     
  • -self, -selves when placed before the article or after the noun or pronoun:
       αὐτὸς ὁ αὐτουργός..... or ὁ αὐτουργὸς αὐτός ....
    the farmer himself does his fair share of the work
     
  • He, and, with the correct endings of course, it becomes, him, his, she, her, it, they, them used as 3rd person pronouns:
    αὐτὴ φέρει αὐτόν
    she (and nobody else) carries him
     
 
 
 
 
 
 
 
 
 
 
 
 

Don't get stuck on the translation "meanwhile". That is only an approximation. A literal translation would be "and in this" meaning something like:
"while this is going on" (here, work in the field)
"while these things are happening"
 
Mother μὲν is preparing supper,
daughter δὲ is setting the table
son δὲ is strumming his guitar
ἐν δὲ τοῦτῳ  the bell rings. It's the police. Why?

I'm up a ladder, paint pot and brush in hand, painting away furiously.
ἐν δὲ τούτῳ the phone rings.....

The little boys are playing cowboys and Indians.
ἐν δὲ τούτῳ it gets dark but they are so engrossed in their game that they do not notice and go on playing
ἐν δὲ τούτῳ their waiting mothers are becoming more and more worried

A young man is waiting for his girl-friend in front of the cinema. She's late again.
ἐν δὲ τούτῳ a taxi draws up and out steps a most gorgeous woman. The young man can't help but stare at her in admiration.
ἐν δὲ τούτῳ his girl friend turns up and sees him staring. Oh dear!
 


 
 
 
 
 
 
 
 
 
 
 
ἐπεὶ δὲ εἰς τὸν ἀγρὸν εἰσβαίνει , καλεῖ....
When he walks into the field, he calls....
ἐπεὶ δὲ ὁ Δικαιόπολις καλεῖ, the slave comes.
ἐπεὶ δὲ ὁ ἥλιος κατατρίβει αὐτούς, ἡσυχάζουσιν.
Contrast this with "while"
ἐν ὧ δὲ φλέγει ὁ ἥλιος , οὐ πονοῦσιν .
ἐπεὶ δὲ ὁ ἥλιος καταδύνει, τοὺς βοῦς λύουσιν.
ἐπεὶ δὲ τοὺς λίθους βλέπει, ὁ Ξανθίας nearly faints.

Practice ἐπεὶ δὲ in English sentences
ἐπεὶ δὲ the mouse is away, the cats are bored etc.
 


 
 
 
 
 
 
 
 
 
 
 
Don't analyze all the time, listen to how it sounds and feel what is implied. If you come across little words like
μέν ....  δέ  
plain δέ  
οὖν  
 καὶ τὰ λοιπά and the rest (i.e. etcetera.)
tell yourself: This is Greek, the Greeks had a reason for using these little words. They found them useful for making their meaning clearer, subtler, more precise, more emphatic, whatever. Try and get a feel for their way of expressing themselves.
Use these little words in English sentences, just to get the hang of them.

I μὲν type and type and type, you δὲ do what?
The μὲν rain falls, the δὲ plants rejoice.
There's a party going on. ἐν δὲ τούτῳ it starts to rain.
The οὖν barbecue is interrupted. The γὰρ charcoal got drenched.
The μεν̀ οὖν garden party is ruined, the δὲ lawn heaves a sigh of relief.
The γὰρ rain gives pleasure to the μὲν flowers, but disappointment to the δὲ people.
You could of course also say:
relief μὲν to the flowers, disappointment δὲ to the people.
It depends on what you contrast or balance

relief and disappointment
or
flowers and people

Feel the difference and use the little words, or at least pay close attention to them in your reading, till you really sense what they imply.  ἐγὼ μὲν γὰρ find them expressive and harmonious, σὺ δὲ practice and you'll get there in no time.