Learn Hardcore Greek: Ο φίλος μου είναι πολύ αστείος.

In Greek, a noun used as a specific person/thing is usually preceded by the definite article. Possessives like μου, σου, etc., do not replace the article.

So:

Ο φίλος μου = my (specific) friend
Η μητέρα μου = my mother
Το σπίτι μου = my house

Leaving out the article (Φίλος μου είναι πολύ αστείος) is possible in some contexts, but it sounds more stylistic/poetic or very colloquial. The safe, standard form is Ο φίλος μου.

Why is it φίλος μου and not μου φίλος like “my friend”?

In Greek, unstressed possessive pronouns (my, your, his, etc.) normally come after the noun:

ο φίλος μου – my friend
η αδερφή σου – your sister
το βιβλίο του – his book

Putting μου in front (μου φίλος) is not standard modern Greek; it can show up in poetry, fixed expressions, or very marked/emotional language, but it’s not the normal pattern you should learn.

So the regular order is: article + noun + possessive.

What exactly is μου? Is it a word by itself or like an ending?

Μου is a separate word: the genitive singular of the 1st person pronoun. Functionally, here it is:

an unstressed (enclitic) possessive pronoun meaning my / of mine.

It doesn’t change form for masculine/feminine/neuter nouns; it’s always μου:

ο φίλος μου – my (male) friend
η φίλη μου – my (female) friend
το παιδί μου – my child

Because it’s unstressed, μου normally does not take an accent mark unless needed for disambiguation in writing.

Why is the verb είναι used and not just “Ο φίλος μου πολύ αστείος”?

Greek generally requires the verb “to be” (είμαι) in sentences like this. You can’t normally drop it the way some languages do.

Ο φίλος μου είναι πολύ αστείος. – correct
Ο φίλος μου πολύ αστείος. – feels incomplete / wrong in standard Greek

Είναι is the 3rd person singular form of είμαι (“to be”), so it matches the subject ο φίλος μου (he).

What are the grammatical forms of φίλος and αστείος here, and why do they look similar?

Both φίλος and αστείος are:

masculine
singular
nominative (subject case)

Greek adjectives must agree with the noun they describe in gender, number, and case. So:

ο φίλος (masc. nom. sg.) → αστείος (masc. nom. sg.)
Ο φίλος μου είναι αστείος.

If the noun changed, the adjective would change too:

η φίλη μου είναι αστεία. – my (female) friend is funny (fem. nom. sg.)
το παιδί μου είναι αστείο. – my child is funny (neut. nom. sg.)
οι φίλοι μου είναι αστείοι. – my (male/mixed) friends are funny (masc. nom. pl.)

Why doesn’t πολύ change form to agree with φίλος or αστείος?

In this sentence πολύ is an adverb meaning very, and adverbs in Greek are invariable: they don’t change for gender, case, or number.

πολύ αστείος – very funny (masculine)
πολύ αστεία – very funny (feminine)
πολύ αστείο – very funny (neuter)
πολύ έξυπνος/έξυπνη/έξυπνο – very smart

When πολύ is used as an adjective meaning much/many, it can have different forms (e.g. πολλή δουλειά, πολλοί άνθρωποι), but here it’s an adverb, so it stays πολύ.

How would I say “My (female) friend is very funny” in Greek?

You change the noun and adjective to the feminine forms:

Η φίλη μου είναι πολύ αστεία.

Changes:

Ο φίλος → Η φίλη (masculine → feminine noun)
αστείος → αστεία (masculine → feminine adjective)

Μου, είναι, and πολύ stay the same.

How do I say “My friends are very funny”?

Use the plural:

Οι φίλοι μου είναι πολύ αστείοι. – My (male or mixed group) friends are very funny.

Form changes:

Ο φίλος → Οι φίλοι (masc. nom. sg. → masc. nom. pl.)
αστείος → αστείοι (masc. nom. sg. → masc. nom. pl.)
είναι happens to be the same form for he/she/it is and they are.

For a group of only female friends, you could say:

Οι φίλες μου είναι πολύ αστείες.

Can I change the word order, like “Ο φίλος μου είναι αστείος πολύ” or “Πολύ αστείος είναι ο φίλος μου”?

Yes, Greek word order is fairly flexible, but the neutral and most common order here is:

Ο φίλος μου είναι πολύ αστείος.

Other orders can add emphasis or sound more poetic/colloquial:

Ο φίλος μου είναι αστείος πολύ. – puts a bit more emphasis on πολύ at the end.
Πολύ αστείος είναι ο φίλος μου. – emphasizes how funny he is; “Very funny is my friend.”

All of these are grammatically possible, but as a learner the safest default is the original order.