Na otoku smo vidjeli kornjaču blizu stijena.

Because na here means on in the sense of location, and when na expresses location, Croatian uses the locative case.

• otok = island
• na otoku = on the island

Compare:

• na otoku = on the island, while being there
• na otok = onto the island, motion toward it

So in this sentence, the action happens there, not toward it, which is why otoku is correct.

Croatian normally uses na with islands:

• na otoku = on the island
• na Braču = on Brač
• na Hvaru = on Hvar

This is just the usual Croatian pattern. Even though English also says on an island, you should not assume u for every place. A good rule is:

• u for many enclosed places, countries, cities, buildings
• na for surfaces, islands, and some other location types

So na otoku is the natural Croatian choice.

Croatian commonly forms the past tense with two parts:

• the auxiliary verb biti = to be
• a past participle

Here:

• smo = we are / auxiliary for we in the past tense
• vidjeli = seen

Together, smo vidjeli means we saw or more literally we have seen.

This is called the perfect tense, and it is the normal way to talk about past actions in everyday Croatian.

The participle agrees with the subject.

• vidjeli = masculine plural, or mixed group, or default plural
• vidjele = feminine plural

So if the group we includes at least one male, or if the gender is not specified, vidjeli is the normal form.

If a group of only women were speaking, they would usually say:

• Na otoku smo vidjele kornjaču blizu stijena.

This is because smo is a clitic. Croatian clitics usually go in the second position of the clause.

So in:

• Na otoku smo vidjeli kornjaču blizu stijena.

the first unit is Na otoku, and then the clitic smo comes after it.

You can think of it like this:

• first element: Na otoku
• second-position clitic: smo
• rest: vidjeli kornjaču blizu stijena

This is very normal Croatian word order.

Because kornjaču is the accusative singular, used for the direct object of vidjeti.

• nominative: kornjača = turtle
• accusative: kornjaču = turtle, as the thing seen

Since we saw the turtle, the turtle is the direct object, so Croatian uses the accusative.

This is very common with feminine nouns ending in -a:

• žena → ženu
• kuća → kuću
• kornjača → kornjaču

Because blizu requires the genitive case, not the dative or instrumental.

So:

• stijene = rocks/cliffs (basic plural form: nominative)
• stijena = of rocks / near rocks (genitive plural)

That is why the sentence uses:

• blizu stijena = near the rocks

This is something worth memorizing: blizu is followed by the genitive.

It looks singular, but here it is actually genitive plural.

For the noun stijena:

• nominative singular: stijena
• nominative plural: stijene
• genitive plural: stijena

So after blizu, you need the genitive plural, and that happens to be the same shape as the singular nominative.

This kind of form overlap is normal in Croatian.

Yes. Croatian word order is more flexible than English, because case endings show what each noun is doing.

The neutral version here is:

• Na otoku smo vidjeli kornjaču blizu stijena.

But you could also hear things like:

• Kornjaču smo vidjeli na otoku blizu stijena.
• Blizu stijena smo na otoku vidjeli kornjaču.

These versions shift emphasis, but the basic meaning stays the same.

Still, not every order sounds equally natural in every context. The original sentence is a smooth, natural choice.

Because Croatian has no articles like English a/an/the.

So a noun like kornjaču can mean:

• a turtle
• the turtle

The exact meaning depends on context.

The same is true for:

• na otoku = on an island / on the island
• blizu stijena = near rocks / near the rocks

Croatian relies on context rather than articles.

The letter č is pronounced roughly like ch in church, but usually a bit firmer.

So:

• kornjaču sounds approximately like kor-nya-chu

Also note:

• nj in kornjaču is like the ny sound in canyon
• j in Croatian is like English y

So kornjača is approximately:

• kor-nya-cha

That pronunciation is much closer than reading j like English j.