Learn Hardcore Greek: Στη θάλασσα κοντά στο νησί κολυμπούν μικρά ψάρια και μεγάλα ψάρια. - In the sea near the island small fish and big fish are swimming.

σε + τη = στη (before most consonants)
σε + την = στην (you’ll often see the ν kept before vowels and some consonants like κ, π, τ, ξ, ψ, μπ, ντ, γκ, τσ, τζ to make pronunciation smoother)

In your sentence:

Στη θάλασσα = in the sea
Here θάλασσα is feminine, so τη is the correct article, and it contracts with σε to give στη.

So it is historically two words (σε τη), but in normal writing and speech it’s treated as one: στη.

Why is it Στη θάλασσα (with “the”) and not something like “σε θάλασσα” (without “the”)?

Greek uses the definite article much more than English, especially with nouns used in a general or generic sense.

Στη θάλασσα literally is in the sea, but depending on context it can mean:
- in the sea (that we’re talking about / can see)
- in the sea (in general, as a place where fish live)

If you say σε θάλασσα without an article, it sounds incomplete or very unusual, like “in some sea (unspecified)” and is generally not what you say for a normal, neutral sentence like this.

So Greek prefers:

Στη θάλασσα κολυμπούν…
In the sea, ... swim ...

even if English might drop the article and say “In the sea” or just “In the ocean / at sea” more loosely.

What is κοντά σε doing grammatically in κοντά στο νησί?

κοντά means near / close (by) and is often followed by σε.

κοντά σε
- noun = near / close to (something)

In your sentence:

κοντά σε + το νησί
→ κοντά στο νησί

Here:

σε
- το = στο (another common contraction)
νησί is neuter singular, so its definite article in the accusative is το.

So:

κοντά στο νησί = near the island / close to the island

Note: Grammatically, σε always takes the accusative case in modern Greek, so το νησί is in accusative.

What is the basic word order here? Could I say Κολυμπούν μικρά ψάρια και μεγάλα ψάρια στη θάλασσα κοντά στο νησί instead?

Yes, you can. Greek word order is relatively flexible compared to English, because the endings on nouns and verbs show who’s doing what.

In your sentence:

Στη θάλασσα κοντά στο νησί κολυμπούν μικρά ψάρια και μεγάλα ψάρια.

The structure is basically:

[Place] [Place-detail] [Verb] [Subject]
Στη θάλασσα (in the sea)
κοντά στο νησί (near the island)
κολυμπούν (they swim)
μικρά ψάρια και μεγάλα ψάρια (small fish and big fish)

You could also say:

Κολυμπούν μικρά ψάρια και μεγάλα ψάρια στη θάλασσα κοντά στο νησί.
Μικρά ψάρια και μεγάλα ψάρια κολυμπούν στη θάλασσα κοντά στο νησί.

All are grammatically correct. The original version puts more emphasis on the location first (“In the sea near the island…”), then tells you what is happening there.

What form is κολυμπούν? Which person and number is it?

κολυμπούν is:

Verb: κολυμπώ / κολυμπάω = to swim
Tense: Present
Person: 3rd person plural (they)

So it literally means they swim or are swimming.

The full present tense of κολυμπώ (formal) is:

εγώ κολυμπώ / κολυμπάω – I swim
εσύ κολυμπάς – you swim
αυτός / αυτή / αυτό κολυμπά – he / she / it swims
εμείς κολυμπάμε – we swim
εσείς κολυμπάτε – you (pl./formal) swim
αυτοί / αυτές / αυτά κολυμπούν – they swim

In everyday speech, you’ll also hear another 3rd plural ending:

κολυμπάνε instead of κολυμπούν.

Both κολυμπούν and κολυμπάνε are correct in modern Greek. Some speakers find κολυμπούν a bit more formal or “written” and κολυμπάνε more colloquial, but both are widely used.

Why do we say μικρά ψάρια και μεγάλα ψάρια and not just μικρά και μεγάλα ψάρια?

Both versions are grammatically correct:

μικρά ψάρια και μεγάλα ψάρια
μικρά και μεγάλα ψάρια

The difference is more about style and emphasis:

μικρά ψάρια και μεγάλα ψάρια
- Repeats the noun ψάρια (“fish”) in both parts.
- Slightly more rhythmic and balanced.
- Can subtly emphasize that we are talking clearly about two groups of fish: small ones and big ones.
μικρά και μεγάλα ψάρια
- More compact.
- Feels a bit more like a single broad group: small and big fish together.

In everyday speech, you’ll often hear the shorter μικρά και μεγάλα ψάρια, but the longer form is also natural and not wrong at all.

Why are the adjectives μικρά and μεγάλα in that particular form? What are they agreeing with?

μικρά and μεγάλα are adjectives meaning small and big.

They are in:

Gender: neuter
Number: plural
Case: nominative

because they must agree with the noun ψάρια:

ψάρι (singular) – neuter
ψάρια (plural) – neuter plural

So:

singular: μικρό ψάρι, μεγάλο ψάρι
plural: μικρά ψάρια, μεγάλα ψάρια

In Greek, adjectives change their endings to match the noun in gender, number, and case. Here the fish are the subject of the verb κολυμπούν, so they are in the nominative plural, and the adjectives follow that: μικρά, μεγάλα.

What case are μικρά ψάρια and μεγάλα ψάρια in, and why?

They are in the nominative plural.

The subject of a verb in Greek is normally in the nominative case.
Here, the things that are doing the swimming are μικρά ψάρια και μεγάλα ψάρια.

So:

μικρά ψάρια και μεγάλα ψάρια κολυμπούν…
(small fish and big fish swim…)

Because they are the doers of the action, they must be nominative, and the adjectives agree with that case.

What exactly is στο in κοντά στο νησί? Is it also a contraction like στη?

Yes. στο is the contraction of σε το.

σε = in / at / to
το = the (neuter, singular, accusative)

So:

σε + το = στο

In your phrase:

κοντά στο νησί = κοντά σε + το νησί

Since νησί is neuter singular, its article in the accusative is το, and with σε it contracts to στο.

You’ll also see:

σε + τον = στον (masculine)
σε + την = στην (feminine, with ν kept in many contexts)

How is this sentence pronounced, especially the stressed syllables?

Here is the sentence, with stressed syllables in CAPS:

Στη ΘΑ-λα-σσα κον-ΤΑ στο νη-ΣΙ κολ-υ-ΜΠΟΥΝ ΜΙ-κρα ΨΑ-ρια και με-ΓΑ-λα ΨΑ-ρια.

More slowly, word by word:

Στη – /sti/ (no extra stress; it’s a short clitic-like word)
θάλασσα – ΘΑ-λα-σσα
κοντά – κον-ΤΑ
στο – /sto/ (also unstressed)
νησί – νη-ΣΙ
κολυμπούν – κο-λυ-ΜΠΟΥΝ
μικρά – μι-ΚΡΑ
ψάρια – ΨΑ-ρια
και – /ke/ or /ce/ (unstressed)
μεγάλα – με-ΓΑ-λα
ψάρια – ΨΑ-ρια

Each content word has exactly one stressed syllable, shown by the written accent mark in Greek (e.g. θάλασσα, κοντά, νησί, κολυμπούν, μικρά, μεγάλα, ψάρια).