Learn Hardcore Greek: Ο φίλος μου είναι πολύ δυναμικός στη δουλειά.

Placing μου before the noun (μου φίλος) is wrong in modern standard Greek. Possessive pronouns like μου, σου, του usually come after the noun they describe and are often used together with the definite article:

το βιβλίο μου = my book
η αδελφή σου = your sister

Why is the article Ο and not Η or ΤΟ?

Greek articles agree with the gender and number of the noun:

ο – masculine singular
η – feminine singular
το – neuter singular

The noun φίλος (friend, male friend) is masculine, so it takes ο:

ο φίλος = the (male) friend
η φίλη = the (female) friend
το παιδί = the child (neuter)

What exactly does μου mean, and why is there no separate word for "my"?

μου is the unstressed (clitic) form of the possessive pronoun εγώ (I). It means my or of me.

Greek usually uses these short, unstressed forms after the noun instead of a separate word before it:

ο φίλος μου = my friend
το σπίτι μου = my house
η μητέρα μου = my mother

There is a stressed form εμένα meaning "me", but you don’t use it for simple possession in this structure. You would not say ο φίλος εμένα for "my friend" in normal speech.

What does δυναμικός mean here? Does it mean “strong” or “energetic”?

δυναμικός is usually closer to:

dynamic, proactive, energetic, driven

In the context πολύ δυναμικός στη δουλειά, it suggests someone who:

takes initiative
is very active and efficient
is full of energy and drive at work

It’s not primarily about physical strength. For “physically strong” you would more typically use δυνατός:

Είναι πολύ δυνατός. = He is very strong.

Why is it δυναμικός and not something like δυναμική or δυναμικό?

Adjectives in Greek agree with the gender, number, and case of the noun they describe.

ο φίλος → masculine singular nominative
So the adjective must be masculine singular nominative too: δυναμικός.

If the subject were feminine or neuter, the adjective would change:

Η φίλη μου είναι πολύ δυναμική στη δουλειά.
My (female) friend is very dynamic at work.
Το αφεντικό μου είναι πολύ δυναμικό στη δουλειά.
My boss is very dynamic at work. (here αφεντικό is neuter in grammar)

What is πολύ doing here? Is it an adjective or an adverb?

Here πολύ is an adverb meaning very:

πολύ δυναμικός = very dynamic

As an adverb, πολύ is invariable (it doesn’t change form):

πολύ δυναμικός (masc.)
πολύ δυναμική (fem.)
πολύ δυναμικό (neut.)

There is also an adjective πολύς / πολλή / πολύ meaning much / many, which does change form:

πολύς χρόνος = much time
πολλή δουλειά = a lot of work
πολλά βιβλία = many books

In the sentence you gave, πολύ is the adverb “very,” not the adjective “much/many.”

What is είναι exactly? Is it “is” or “are”? Why this form?

είναι is a form of the verb είμαι (to be). It’s used for:

he / she / it is (3rd person singular)
they are (3rd person plural)

The present tense of είμαι is:

εγώ είμαι – I am
εσύ είσαι – you are (singular)
αυτός/αυτή/αυτό είναι – he/she/it is
εμείς είμαστε – we are
εσείς είστε / είσαστε – you are (plural / polite)
αυτοί/αυτές/αυτά είναι – they are

In your sentence, the subject is Ο φίλος μου (he), so είναι means is.

Why is it στη δουλειά and not σε η δουλειά?

στη is a contracted form of:

σε + τη(ν) → στη(ν)

So:

σε = in / at / to
τη (ν) = the (feminine accusative singular)

δουλειά is a feminine noun, and after σε, the noun goes into the accusative case, so:

σε τη δουλειά → στη δουλειά = at work / in the job

Similarly:

στο σπίτι = σε + το σπίτι (at home)
στην πόλη = σε + την πόλη (in the city; here the final -ν is kept)

Why is δουλειά in that form? What case is it, and why?

δουλειά here is in the accusative singular.

In Greek, the preposition σε (“in / at / to”) is normally followed by the accusative:

σε
- δουλειά (accusative) → στη δουλειά

Examples:

Πηγαίνω στη δουλειά. = I go to work.
Είμαι στη δουλειά. = I am at work.

So the case is determined by the preposition σε.

Can I change the word order, for example: Ο φίλος μου στη δουλειά είναι πολύ δυναμικός?

Yes, Greek word order is relatively flexible.

Ο φίλος μου είναι πολύ δυναμικός στη δουλειά.
Neutral: “My friend is very dynamic at work.”
Ο φίλος μου στη δουλειά είναι πολύ δυναμικός.
This often sounds like you are specifying which friend:
“My friend at work is very dynamic.” (i.e. not some other friend)

So the changed word order can slightly shift the emphasis or meaning (from “about my friend, at work he is dynamic” to “the friend I have at work is dynamic”).

Could I say στην δουλειά instead of στη δουλειά?

Yes, both forms are used:

στη δουλειά
στην δουλειά

The -ν at the end of στη(ν) is often:

kept before vowels and some consonants (π, τ, κ, μπ, ντ, γκ, ξ, ψ)
omitted in other cases

In modern everyday writing and speech, many speakers say:

στη δουλειά (without -ν)

But στην δουλειά is also acceptable and not wrong. It’s largely a matter of style and habit here.