Learn Hardcore Greek: Ο αδερφός μου είναι μικρός αλλά πολύ έξυπνος.

Ο αδερφός μου = the brother my (literally) → my brother
It is masculine because αδερφός is a masculine noun.
It is nominative because this noun phrase is the subject of the verb είναι (is).

Other forms of the definite article for comparison:

Masculine: ο (nom.), τον (acc.), του (gen.)
Feminine: η (nom.), την (acc.), της (gen.)
Neuter: το (nom./acc.), του (gen.)

Why is it αδερφός μου and not something like μου αδερφός, and why is there no separate word for my?

In Greek, the normal way to express possession with close relationships, body parts, etc., is:

[definite article] + [noun] + [weak possessive pronoun]

So:

ο αδερφός μου = the brother my → my brother

The possessive pronouns like μου, σου, του, της, μας, σας, τους (my, your, his/her, our, your, their) are clitics: they are short, unstressed forms that normally come after the noun, not before it.

You cannot say μου αδερφός in standard Greek.

If you want to emphasize my, you can use a different structure:

ο δικός μου αδερφός = my own brother / my brother (not someone else’s)

But the default, neutral way is exactly what you see in the sentence: ο αδερφός μου.

What is the difference between αδερφός and αδελφός? Which one should I use?

Both αδερφός and αδελφός mean brother and are pronounced the same: [aðerˈfos] (th as in this).

αδελφός: more traditional/standard spelling, more common in formal writing, in dictionaries, in the Bible, etc.
αδερφός: more colloquial/spoken spelling, extremely common in everyday modern Greek.

Both are correct. In casual speech and informal writing, αδερφός is very common. In more formal or carefully edited texts, you will often see αδελφός.

What grammatical form is αδερφός here, and how would it change in other cases?

In the sentence Ο αδερφός μου είναι μικρός αλλά πολύ έξυπνος, αδερφός is:

Masculine
Singular
Nominative (subject of the verb είναι)

The main forms of αδερφός are:

Nominative (subject):
ο αδερφός → Ο αδερφός μου είναι μικρός.
My brother is young.
Accusative (direct object):
τον αδερφό → Βλέπω τον αδερφό μου.
I see my brother.
Genitive (of …):
του αδερφού → Το σπίτι του αδερφού μου.
My brother’s house.
Vocative (addressing someone):
αδερφέ! → Έλα εδώ, αδερφέ!
Come here, brother!

How do μικρός and έξυπνος work grammatically in this sentence?

Both μικρός and έξυπνος are adjectives describing ο αδερφός μου. In Greek, adjectives must agree with the noun in:

Gender: masculine (because αδερφός is masculine)
Number: singular
Case: nominative (because they describe the nominative subject)

That’s why we have:

ο αδερφός μου → μικρός, έξυπνος (masc. sg. nom.)

If the noun were:

Feminine: η αδερφή μου → μικρή, έξυπνη
Neuter: το παιδί μου → μικρό, έξυπνο
Plural masculine: οι αδερφοί μου → μικροί, έξυπνοι

Why do we need είναι? Could we just say Ο αδερφός μου μικρός αλλά πολύ έξυπνος?

Greek, like English, normally needs a linking verb (copula) between the subject and a predicate adjective:

Ο αδερφός μου είναι μικρός.
My brother is young/small.

είναι is the 3rd person singular form of the verb είμαι (to be).

Just saying Ο αδερφός μου μικρός is not standard; it sounds telegraphic or like a note, not a normal sentence. In everyday correct speech and writing, you need είναι (or another form of είμαι) to form such sentences.

Very briefly, the present of είμαι:

είμαι – I am
είσαι – you are
είναι – he/she/it is
είμαστε – we are
είστε – you are
είναι – they are

Why is there no article before μικρός or έξυπνος, and how would the meaning change if there were one?

In this sentence, μικρός and έξυπνος are predicate adjectives (they describe the subject via the verb είναι), so they normally do not take an article:

Ο αδερφός μου είναι μικρός.
My brother is young/small.

If you add an article, you usually change the meaning from description to identification:

Ο αδερφός μου είναι ο μικρός.
Literally: My brother is the small one / the younger one.
→ You’re picking him out from a group; he is the younger/small one.
Ο αδερφός μου είναι ο έξυπνος.
My brother is the smart one.

So:

without article: what he is like (his qualities)
with article: which one he is (identifying him as “the small one”, “the smart one”, etc.)

Does μικρός mean small or young here? How do I say younger brother or little brother?

μικρός literally means small, but when talking about people—especially family members—it very often means young or younger, like little in English.

In this sentence, Ο αδερφός μου είναι μικρός most naturally means:

My brother is (still) young
or
My little/younger brother is young, depending on context.

Useful distinctions:

ο μικρός μου αδερφός
My little / younger brother (very common, affectionate/colloquial)
ο μικρότερος αδερφός μου
My younger brother (comparative, clearly “the younger one”)
είναι μικρός για την ηλικία του
He is small for his age (physical size)
είναι πολύ μικρός
He is very young / too young (depending on context)

So μικρός can refer to age, size, or both, depending on the situation. Here, with έξυπνος, it naturally suggests age: young but very smart.

What does πολύ do in πολύ έξυπνος? Is it an adjective or an adverb?

In πολύ έξυπνος, πολύ is an adverb meaning very. It modifies the adjective έξυπνος:

πολύ έξυπνος = very smart

As an adverb, πολύ:

Does not change form.
Comes before the adjective or adverb it modifies:
- πολύ καλός – very good
- πολύ μεγάλος – very big
- τρέχει πολύ γρήγορα – he runs very fast

There is also an adjective πολύς / πολλή / πολύ meaning much / many / a lot of. That one does change form:

πολύς χρόνος – much time
πολλή δουλειά – a lot of work
πολλά παιδιά – many children

So:

πολύ έξυπνος (adverb + adjective) → very smart
πολύς κόσμος (adjective + noun) → a lot of people

What is the role of αλλά here, and how is it different from όμως or μα?

αλλά is a coordinating conjunction meaning but. It contrasts two statements:

Ο αδερφός μου είναι μικρός αλλά πολύ έξυπνος.
My brother is young but very smart.

It behaves much like English but, usually connecting two clauses or two adjectives/phrases.

Comparison with similar words:

αλλά – the most neutral, standard but, used everywhere.
μα – also but, more emotional/colloquial and often in exclamations:
Μα τι λες; – But what are you saying?!
όμως – closer to however / nevertheless; more flexible in position:
- Είναι μικρός, όμως είναι πολύ έξυπνος.
  He is young; however, he is very smart.
- Είναι όμως πολύ έξυπνος. – However, he is very smart.

In this simple sentence, αλλά is exactly the natural choice.

How would the sentence change if I were talking about my sister instead of my brother?

You must change the noun and all words that agree with it from masculine to feminine:

Ο αδερφός μου είναι μικρός αλλά πολύ έξυπνος.
My brother is young but very smart.

→

Η αδερφή μου είναι μικρή αλλά πολύ έξυπνη.
My sister is young but very smart.

Changes:

Ο → Η (feminine article)
αδερφός → αδερφή (feminine noun)
μικρός → μικρή (feminine adjective)
έξυπνος → έξυπνη (feminine adjective)

μου and είναι stay the same.

How do you pronounce Ο αδερφός μου είναι μικρός αλλά πολύ έξυπνος?

Approximate pronunciation with stress in capitals (English-style):

Ο – o (as in or, short)
αδερφός – a-dher-FOS (dh like th in this)
μου – moo
είναι – EE-neh
μικρός – mi-KROS
αλλά – a-LA
πολύ – po-LEE
έξυπνος – EX-ip-nos

All together:

o aðer-FOS mu EE-ne mi-KROS a-LA po-LEE EX-ip-nos

In IPA:

[o aðerˈfos mu ˈine miˈkros aˈla poˈli ˈeksipnos]

Stress in Greek is always marked by the written accent on one syllable of each word:
αδερφός, μικρός, αλλά, πολύ, έξυπνος.