Learn Hardcore Greek: Εγώ δουλεύω στο γραφείο, ενώ ο αδερφός μου μένει σπίτι. - I work at the office, while my brother stays at home.

Δουλεύω στο γραφείο. = I work at the office. (no Εγώ, still clear)
Εγώ δουλεύω στο γραφείο. adds emphasis, like:
- I work at the office (as opposed to someone else).

So in this sentence, Εγώ contrasts with ο αδερφός μου:

Εγώ work at the office, while my brother stays at home.

You use Εγώ when you want that contrast or emphasis; otherwise you usually drop it.

What does δουλεύω mean exactly? Is it the normal verb for “to work”?

Yes, δουλεύω is the everyday, most common verb for “to work”:

δουλεύω στο γραφείο – I work at the office
δουλεύεις πολύ – you work a lot

There is also εργάζομαι, which is:

more formal or official (you’ll see it in news, documents, announcements)
usually not used in casual conversation

So:

In normal speech: δουλεύω
In formal written Greek / official style: εργάζομαι

What exactly is στο in στο γραφείο? Is it one word or two?

Στο is a contraction of:

σε (preposition “in / at / to”)
το (neuter singular definite article “the”)

So:

σε + το γραφείο → στο γραφείο = at the office / to the office

Other common contractions:

σε + τον → στον (masc. sing.): στον γιατρό (to the doctor)
σε + την → στη(ν) (fem. sing.): στη δουλειά (at work)
σε + τα → στα (neut. pl.): στα γραφεία (at the offices)

You almost always use the contracted forms (στο, στον, στη, στα) in speech and writing.

Why does γραμείο have the article (το γραφείο), but σπίτι is just σπίτι here, without το?

Good observation; both can take an article, but the usage is different here:

στο γραφείο = literally “at the office”, a specific place (your workplace).
μένει σπίτι is a set pattern meaning “stays at home” in a general sense.

Compare:

Μένει σπίτι. – He stays (at) home.
Μένει στο σπίτι. – He stays in the house (more concrete, this particular house).

So σπίτι without the article often functions almost like an adverb (like “home” in English):

Πάω σπίτι. – I’m going home.

With the article, it’s more literally “the house”:

Πάω στο σπίτι. – I’m going to the house.

What does ενώ mean here? Is it “while” or “whereas”?

Ενώ can mean both, depending on context:

Temporal “while” (two actions happening at the same time)
- Ενώ μαγειρεύω, ακούω μουσική.
  While I’m cooking, I listen to music.
Contrast “whereas / while / but” (contrasting two facts)
- Εγώ δουλεύω στο γραφείο, ενώ ο αδερφός μου μένει σπίτι.
  I work at the office, whereas my brother stays at home.

In your sentence, the main function is contrast, not just time: one person is working, the other is staying home. English could translate it as “while” or “whereas”.

Why is there a comma before ενώ? Is that always necessary?

Yes, you normally put a comma before ενώ when it introduces a second clause that contrasts or adds information to the first:

Εγώ δουλεύω στο γραφείο, ενώ ο αδερφός μου μένει σπίτι.

This is similar to a comma before “while/whereas” in English when it connects two independent clauses.

If ενώ introduces a small dependent clause tightly attached to the main clause, some writers might skip the comma, but in a sentence like this, the comma is standard and correct.

How does the possessive work in ο αδερφός μου? Why after the noun and not before, like in English?

Greek possessives are formed differently from English:

Structure: article + noun + possessive pronoun
- ο αδερφός μου = my brother
- η μητέρα σου = your mother
- το βιβλίο του = his book

Key points:

The possessive pronoun (μου, σου, του, της, μας, σας, τους)
- always comes after the noun.
- does not change form (it’s invariable).
You normally use the definite article with family members and possessed things:
- ο πατέρας μου – my father
- το σπίτι μας – our house

So ο αδερφός μου literally is “the brother of me”, but in normal English we say “my brother”.

What person and tense are the verbs δουλεύω and μένει? Why are they different?

Both verbs are in the present tense, but with different persons:

δουλεύ-ω → 1st person singular: I work
μέν-ει → 3rd person singular: he/she/it stays / lives

They are different because the subjects are different:

(Εγώ) δουλεύω – I work
ο αδερφός μου μένει – my brother stays

Greek doesn’t usually need explicit subject pronouns because the verb endings (-ω, -εις, -ει, etc.) show the person.

Can I change the word order to Εγώ στο γραφείο δουλεύω or ο αδερφός μου σπίτι μένει?

You can change the word order in Greek more freely than in English, but the neutral order here is:

Εγώ δουλεύω στο γραφείο
ο αδερφός μου μένει σπίτι

Other orders are possible, but they change emphasis:

Στο γραφείο δουλεύω εγώ.
- Emphasis on στο γραφείο and εγώ; roughly:
  It’s at the office that I work.
Σπίτι μένει ο αδερφός μου.
- Emphasis on σπίτι:
  It’s at home that my brother stays.

Your versions (Εγώ στο γραφείο δουλεύω, ο αδερφός μου σπίτι μένει) are understandable, but they sound a bit marked/stylized. For a learner, stick to:

[Subject] + [verb] + [rest] as the default pattern.

What does μένει mean here exactly—“stays” or “lives”?

The verb μένω can mean both “to stay” and “to live / reside”, depending on context:

Μένω στο Παρίσι. – I live in Paris.
Σήμερα μένω σπίτι. – Today I’m staying at home.

In your sentence:

ο αδερφός μου μένει σπίτι
can be understood as:
my brother stays at home (as opposed to going out / working)

Without more context, the most natural reading here (since it’s contrasted with δουλεύω στο γραφείο) is “stays at home”, not “lives at home”.

Is αδερφός the same as αδελφός? I’ve seen both.

Yes, they are essentially the same word: both mean “brother”.

αδελφός is the more traditional / formal / standard spelling.
αδερφός is a common modern spelling that reflects how many people actually pronounce it.

In everyday modern Greek you will see both forms, and they are both understood and accepted. They are pronounced the same in normal speech.

Why is the pronoun μου unstressed and written after the noun, not capitalized or emphasized?

Μου here is a clitic (an unstressed little word) that “leans on” the noun:

ο αδερφός μου – my brother

Features of clitics like μου in this use:

They are unstressed in pronunciation.
They come directly after the word they belong to (here, the noun).
They are written in lowercase and not separated by a comma.

You would only stress or isolate μου in speech for strong emphasis, for example in a contrast:

Είναι φίλος ΜΟΥ, όχι δικός σου.
He is MY friend, not yours.

But in your sentence it’s just the normal, unstressed possessive: ο αδερφός μου.