Na jezeru plivaju patka, guska i bijeli labud.

Because na here expresses location, not movement.

• na jezeru = on/at the lake → static location
• na jezero = onto the lake → movement toward a place

After na, Croatian uses:

• locative for location
• accusative for motion toward something

So jezeru is the locative singular of jezero.

Because it is in the locative case after na when talking about location.

The noun is:

• nominative: jezero = lake
• locative: jezeru = on/at the lake

This is a normal case change in Croatian. A lot of neuter nouns in -o change to -u in the locative singular.

Because together they form a compound subject:

• patka
• guska
• bijeli labud

Even though each noun is singular, the whole group means more than one animal, so the verb is plural:

• pliva = he/she/it swims
• plivaju = they swim

This works like English: The duck, the goose, and the swan are swimming, not is swimming.

They are in the nominative case because they are the subject of the sentence.

Basic forms here are:

• patka = duck
• guska = goose
• labud = swan

And the adjective also matches the noun:

• bijeli labud = white swan

So these words stay in nominative because they are the ones doing the action.

Because adjectives in Croatian must agree with the noun they describe.

Labud is:

• masculine
• singular
• nominative

So the adjective must also be:

• masculine
• singular
• nominative

That gives:

• bijeli labud

If the noun changed case, number, or gender, the adjective would change too.

Croatian normally does not use articles like English a/an/the.

So:

• patka can mean a duck or the duck
• guska can mean a goose or the goose
• bijeli labud can mean a white swan or the white swan

The exact meaning depends on context.

This is very normal in Croatian, and learners from English often notice it right away.

Croatian word order is more flexible than English word order.

This sentence starts with Na jezeru to put the location first. That can sound natural if the speaker wants to set the scene first:

• Na jezeru plivaju patka, guska i bijeli labud.

A more neutral version is also possible:

• Patka, guska i bijeli labud plivaju na jezeru.

Both are grammatical. The difference is mainly one of focus and style, not basic meaning.

Yes, but it is not exactly the same nuance.

• na jezeru usually means on the lake / at the lake, often focusing on the lake as a place or surface area
• u jezeru means in the lake, focusing more on being inside the water

Since swimming happens in water, u jezeru is possible, but na jezeru is also very natural when talking about something happening on a lake as a location.

So the choice depends a bit on what you want to emphasize.

Croatian allows that quite easily, especially when another element comes first.

Here the structure is:

• Na jezeru = location
• plivaju = verb
• patka, guska i bijeli labud = subject

This is not strange in Croatian. English usually prefers the subject before the verb, but Croatian can move things around more freely because case endings show the grammatical roles.

Yes. Plivaju is the 3rd person plural present tense of plivati (to swim).

Useful forms:

• plivam = I swim
• plivaš = you swim
• pliva = he/she/it swims
• plivamo = we swim
• plivate = you all swim
• plivaju = they swim

Here it matches the plural subject group.

Because it is a list.

Croatian punctuation here works much like English:

• items in a list are separated by commas
• the last two items are joined by i = and

So:

• patka, guska i bijeli labud = duck, goose, and white swan

Unlike English, Croatian usually does not use an Oxford comma before i.

Yes, absolutely.

That version may feel a little more straightforward to an English speaker because the subject comes first.

Compare:

• Na jezeru plivaju patka, guska i bijeli labud.
• Patka, guska i bijeli labud plivaju na jezeru.

Both are correct. The first highlights the setting first; the second highlights the animals first.

Yes. In Croatian, grammatical gender is not the same thing as natural sex.

The noun labud is grammatically masculine, so it takes masculine agreement:

• bijeli labud

That does not automatically mean the swan is biologically male. It is just the grammatical gender of the word.

The same idea exists with many Croatian nouns for animals and objects.