Learn Hardcore Greek: Η θεία μου λέει ότι η σκηνή που της αρέσει πιο πολύ στο θέατρο είναι όταν όλοι τραγουδάνε μαζί. - My aunt says that the scene she likes most in the theatre is when everyone sings together.

Putting μου before the noun (η μου θεία) is ungrammatical in modern Greek (except in some very old or poetic styles).
So the normal order is: article + noun + possessive pronoun.

What exactly does λέει ότι mean here, and how is ότι used?

λέει ότι means “(she) says that …”.

λέει = she says
ότι = that (conjunction introducing a clause)

So:
Η θεία μου λέει ότι… = My aunt says that…

A few points:

ότι introduces a reported statement, like English that in She says that…
You will also see πως used in the same way: λέει πως… ≈ λέει ότι…
In everyday speech, ότι/πως is often dropped, just like in English:
- Η θεία μου λέει η σκηνή… (colloquial) ≈ My aunt says (that) the scene…

In careful written Greek, ότι (or πως) is normally kept.

Why is it η σκηνή που της αρέσει instead of something like “η σκηνή που αυτή αρέσει” for “the scene she likes”?

The verb αρέσει does not work like English to like.

In Greek, the thing liked is the subject, and the person who likes it is expressed in the genitive with a weak pronoun (or with σε + accusative):

Η σκηνή της αρέσει.
Literally: The scene is pleasing *to her.
Meaning: *She likes the scene.

So in the relative clause:

η σκηνή που της αρέσει
= the scene that is pleasing to her
= the scene that she likes

You cannot say η σκηνή που αυτή αρέσει; αρέσει needs the person in the genitive (της) or with σε:

η σκηνή που της αρέσει (correct)
η σκηνή που αρέσει σε αυτήν (also correct, more emphatic)

What is the role of που in η σκηνή που της αρέσει πιο πολύ?

Here που is a relative pronoun, like English that/which/who introducing a relative clause:

η σκηνή που της αρέσει πιο πολύ
= the scene *that she likes the most*

You could also use the more formal η οποία:

η σκηνή η οποία της αρέσει πιο πολύ

In everyday modern Greek, που is by far the most common relative pronoun and often replaces ο οποίος/η οποία/το οποίο.

Why is the pronoun της before αρέσει (in που της αρέσει) and not after, like που αρέσει της?

The short (clitic) object pronouns in Greek have fairly strict positions:

With a simple verb, they normally go before the verb:
- της αρέσει (not αρέσει της)
- τον βλέπω (I see him)
- μου είπε (he told me)

After the verb is possible only in some special cases (e.g. imperatives: πες μου! “tell me!”).
So in this sentence, the standard neutral order is:

που της αρέσει = that to‑her is‑pleasing

What does πιο πολύ mean here, and how is it different from περισσότερο?

πιο πολύ literally means “more much”, but together it just means “more”.
In this context it functions like “the most”, because it’s inside a phrase with a definite noun (η σκηνή = the scene).

η σκηνή που της αρέσει πιο πολύ
= the scene that she likes more
≈ the scene she likes the most

You can usually substitute περισσότερο:

η σκηνή που της αρέσει περισσότερο

Both πιο πολύ and περισσότερο are very common.
πιο πολύ is slightly more colloquial/neutral; περισσότερο can feel a bit more “careful” or formal in some contexts, but they’re often interchangeable.

Why do we say στο θέατρο and not σε το θέατρο or just σε θέατρο?

Greek normally contracts the preposition σε with the definite article:

σε + το → στο
σε + τα → στα
σε + την → στη(ν)
σε + τους → στους, etc.

So:

σε το θέατρο → στο θέατρο

You would use σε θέατρο (without article) only if you specifically want to say in a theater (any theater) in a very generic or indefinite sense. Here, στο θέατρο is more like:

in the theater / at the theater (as a general place or context, or “in the play” depending on context)

What is the structure of είναι όταν όλοι τραγουδάνε μαζί? Why is είναι needed?

The phrase:

η σκηνή … είναι όταν όλοι τραγουδάνε μαζί

literally is:

the scene … *is when everyone sings together*

Here:

είναι = is (linking verb / copula)
όταν όλοι τραγουδάνε μαζί = a clause functioning as the complement of είναι (telling us what the scene is)

So structurally:

η σκηνή … είναι [όταν όλοι τραγουδάνε μαζί]

You need είναι to link the subject (η σκηνή…) to its complement (όταν όλοι τραγουδάνε μαζί).
Without είναι, the sentence would be incomplete or ungrammatical.

What is the difference between τραγουδάνε and τραγουδούν? Are both correct?

Both are correct 3rd person plural forms of the present tense of τραγουδάω / τραγουδώ (to sing):

τραγουδάνε – more colloquial / very common in speech
τραγουδούν – more formal / more common in writing

So:

όλοι τραγουδάνε μαζί
όλοι τραγουδούν μαζί

both mean “everyone sings together”.
In everyday spoken Greek, you will hear τραγουδάνε more often.

Can the word order όταν όλοι τραγουδάνε μαζί be changed, for example to όταν τραγουδάνε όλοι μαζί?

Yes. Greek allows quite flexible word order, especially with adverbs and quantifiers like όλοι.

All of these are natural:

όταν όλοι τραγουδάνε μαζί
όταν τραγουδάνε όλοι μαζί
όταν όλοι μαζί τραγουδάνε

They all mean “when everyone sings together”.
The differences are very slight (focus/intonation), and for most purposes you can treat them as equivalent.