Galeb leti nad mirnim morem.

Galeb is in the nominative singular because it is the subject of the sentence — the thing doing the action.

• galeb = seagull
• It is a masculine noun
• In this sentence, it means the seagull or a seagull, depending on context

Croatian usually does not use articles like the or a, so galeb can mean either one.

Leti is the 3rd person singular present tense of the verb letjeti (to fly).

So:

• ja letim = I fly / am flying
• ti letiš = you fly / are flying
• on/ona/ono leti = he/she/it flies / is flying

Because galeb is singular, the verb is also singular: galeb leti.

Because after nad in this sentence, Croatian uses the instrumental case.

• basic form: more = sea
• instrumental singular: morem

So:

• nad morem = above the sea / over the sea

Here the sentence describes where the seagull is flying, not movement onto or over something as a destination, so instrumental is used.

Because the adjective mirnim has to agree with morem in:

• case: instrumental
• number: singular
• gender: neuter

The noun is:

• more = neuter singular
• instrumental singular = morem

So the adjective miran (calm) becomes:

• mirnim to match morem

That is why you get:

• nad mirnim morem = above the calm sea

No. Nad can take two different cases, depending on meaning:

• instrumental for location: above / over
• accusative for movement toward a position above something

In this sentence:

• Galeb leti nad mirnim morem.
• The meaning is basically The seagull is flying above the calm sea
• So Croatian uses instrumental

A useful contrast:

• Ptica leti nad morem. = The bird is flying over the sea.
• Ptica leti nad more. = The bird is flying to a position over the sea.

In everyday learning, the key idea is:

• where? → often instrumental
• to where? → often accusative

Croatian has no articles like the or a/an.

So galeb can mean:

• a seagull
• the seagull

and more can mean:

• a sea
• the sea

The exact meaning depends on context, not on an article.

That is very common in Croatian, and English speakers often need time to get used to it.

Yes. Croatian word order is more flexible than English because the noun endings show grammatical roles.

The neutral order here is:

• Galeb leti nad mirnim morem.

But other orders are possible, for example:

• Nad mirnim morem leti galeb.
• Leti galeb nad mirnim morem.

These alternatives may sound more poetic, emphatic, or context-driven, but they are still understandable because the endings make the structure clear.

It can mean either, depending on context.

Croatian present tense often covers both:

• Galeb leti. = The seagull flies.
• Galeb leti. = The seagull is flying.

English forces you to choose more often than Croatian does. Croatian leaves that distinction to context unless there is a reason to make it clearer.

Here miran means calm or peaceful.

So mirno more means:

• calm sea
• a calm sea

In this sentence it changes form to mirnim because of case agreement:

• nominative: mirno more
• instrumental: mirnim morem

A simple approximation is:

• Galeb leti nad mirnim morem
• GAH-leb LEH-tee nahd MEER-neem MOH-rem

A few helpful notes:

• g is always a hard g as in go
• e is like e in met
• i is like ee in see
• r in Croatian is usually trilled or tapped
• stress can vary by dialect, but the approximation above is good enough for a learner

The dictionary forms are:

• galeb = seagull
• letjeti = to fly
• nad = above / over
• miran = calm
• more = sea

In the sentence, some words are changed from their dictionary form because Croatian uses cases and verb conjugation:

• letjeti → leti
• miran → mirnim
• more → morem