Ο αδερφός μου είναι μικρός αλλά πολύ έξυπνος.

to be

Breakdown of Ο αδερφός μου είναι μικρός αλλά πολύ έξυπνος.

• πολύ: very
• μου: my
• αλλά: but

Questions & Answers about Ο αδερφός μου είναι μικρός αλλά πολύ έξυπνος.

What is the function of Ο in this sentence?

Ο is the masculine singular definite article in the nominative case. It corresponds roughly to the in English.

• 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.