Learn Hardcore Greek: Το δωμάτιο του αδερφού μου είναι πάντα ακατάστατο, αλλά το δικό μου είναι συνήθως τακτοποιημένο. - My brother’s room is always messy, but mine is usually tidy.

So the structure is: το δωμάτιο του αδερφού μου = the room of my brother.
Greek often uses the article + noun (in genitive) + μου to express possession: το βιβλίο του φίλου μου = my friend’s book.

Why is it αδερφού and not αδερφός?

αδερφός is the nominative form (the basic dictionary form, used for the subject):

Ο αδερφός μου είναι ψηλός. = My brother is tall.

In του αδερφού μου, αδερφού is genitive singular:

nominative: ο αδερφός (the brother)
genitive: του αδερφού (of the brother)

We need the genitive because we’re expressing possession: το δωμάτιο του αδερφού μου = the room of my brother.

Is there a difference between αδερφός and αδελφός?

They are essentially the same word:

αδερφός – more common in everyday, informal speech and writing.
αδελφός – more formal / standard spelling, often seen in written or more formal contexts.

Meaning and grammar are the same: both mean brother and decline the same way.

Why is it το δωμάτιο? Is δωμάτιο always neuter?

Yes, δωμάτιο (room) is a neuter noun, so it takes the neuter article το in the singular:

το δωμάτιο = the room
τα δωμάτια = the rooms

Adjectives that describe δωμάτιο must also be neuter singular:
το δωμάτιο είναι ακατάστατο / τακτοποιημένο.

Why do the adjectives end in -ο: ακατάστατο, τακτοποιημένο?

They agree in gender, number, and case with δωμάτιο, which is neuter singular.

Typical adjective pattern:

masculine: ακατάστατος
feminine: ακατάστατη
neuter: ακατάστατο

Same for τακτοποιημένος / τακτοποιημένη / τακτοποιημένο.

Since το δωμάτιο is neuter singular, we use ακατάστατο, τακτοποιημένο.

What exactly is the difference in meaning between ακατάστατο and τακτοποιημένο?

Both are adjectives describing the state of the room:

ακατάστατο = messy, untidy, things are all over the place.
τακτοποιημένο = tidy, in order, organized, things are put where they belong.

So the sentence contrasts two opposite states: always messy vs usually tidy.

What does δικό μου add that just μου wouldn’t?

μου by itself simply means my:

το δωμάτιό μου = my room.

δικό μου is an emphatic possessive (“my own / mine”), and it agrees in gender with the thing possessed:

το δικό μου (δωμάτιο) = my own (room) / mine (neuter)
η δική μου (αδερφή) = my own sister (feminine)
ο δικός μου (φίλος) = my own friend (masculine)

In the sentence, το δικό μου stands for το δικό μου δωμάτιο (“my own room / mine”) and emphasizes the contrast with του αδερφού μου.

How would I normally say “my room” in Greek, without this emphasis?

The most neutral way is:

το δωμάτιό μου = my room.

Structure: το (the) + δωμάτιο (room) + μου (my).

You could also say το δικό μου δωμάτιο, but that has extra emphasis, like “my own room” (often used when contrasting with someone else’s room, as in this sentence).

Could I say “…αλλά το δωμάτιό μου είναι συνήθως τακτοποιημένο” instead of “…αλλά το δικό μου είναι συνήθως τακτοποιημένο”?

Yes, that is perfectly correct:

Το δωμάτιο του αδερφού μου είναι πάντα ακατάστατο, αλλά το δωμάτιό μου είναι συνήθως τακτοποιημένο.

This version:

repeats δωμάτιο explicitly,
uses the simple possessive μου without special emphasis.

The original το δικό μου is a bit more contrastive and more natural in this kind of comparison, similar to English “…but mine is usually tidy.”

What is the nuance between πάντα and συνήθως?

Both are adverbs of frequency:

πάντα = always (100% of the time; or at least, that’s the speaker’s exaggeration)
συνήθως = usually, generally, most of the time (high frequency but not always)

So the sentence is saying:

The brother’s room: always messy
My room: usually tidy (not claiming perfection!)

Where can I put πάντα and συνήθως in the sentence? Can the word order change?

Common positions are:

Το δωμάτιο του αδερφού μου είναι πάντα ακατάστατο.
Το δωμάτιο του αδερφού μου πάντα είναι ακατάστατο. (possible, but less neutral)
Το δωμάτιο του αδερφού μου είναι ακατάστατο πάντα. (sounds more marked / emphatic)

The most natural, neutral order in modern Greek is usually:

[subject] + είναι + [adverb] + [adjective]

So the original είναι πάντα ακατάστατο and είναι συνήθως τακτοποιημένο are very typical.

Why is είναι used twice? Could I omit the second είναι?

In Greek, each clause normally needs its own verb, so:

…είναι πάντα ακατάστατο, αλλά το δικό μου είναι συνήθως τακτοποιημένο.

You cannot naturally drop the second είναι here.
In English you might say: “My brother’s room is always messy, but mine usually tidy.” (omitting “is”) in some informal styles, but in Greek that would sound incorrect or at least very odd. Each clause keeps its own είναι.