Kapetan kaže da ćemo stići na otok prije mraka.

Here da means that and introduces a subordinate clause:

Kapetan kaže da ćemo stići... = The captain says that we will arrive...

In English, that is often omitted:

• The captain says we will arrive...

In Croatian, da is usually kept in this kind of sentence.

Ćemo means we will.

It is the 1st person plural future auxiliary, from the verb htjeti (to want), which is also used to form the future tense.

So:

• ćemo = we will
• stići = arrive / reach

Together:

• ćemo stići = we will arrive

Because in a da-clause, the normal order is:

da + auxiliary + infinitive

So:

• da ćemo stići = correct, natural
• da stići ćemo = not correct here

In an independent sentence, you can very often say:

• Stići ćemo prije mraka. = We will arrive before dark.

But after da, da ćemo stići is the standard pattern.

That is a very common question. Croatian na often does mean on, but with some destinations it is also used where English uses to.

With otok (island), Croatian normally says:

• na otok = to the island
• na otoku = on the island

So Croatian treats island as a place you go onto, not just into.

Compare:

• Idemo na otok. = We’re going to the island.
• Mi smo na otoku. = We are on the island.

Because na can take different cases depending on meaning.

Here it expresses movement toward a destination, so it takes the accusative:

• na otok = to the island

If it expressed location, it would take the locative:

• na otoku = on the island

So:

• movement → na + accusative
• location → na + locative

Mraka is genitive singular.

The preposition prije (before) requires the genitive:

• prije mraka = before dark
• prije puta = before the trip
• prije ručka = before lunch

The base noun is:

• mrak = darkness / dark

Its genitive singular is:

• mraka

Literally, yes, it is before darkness, but in natural English it usually means:

• before dark
• before nightfall

So it is a very normal time expression, not something poetic or strange.

Kaže is 3rd person singular present: he/she says.

Here:

• kapetan = the captain
• kaže = says

So:

• Kapetan kaže... = The captain says...

This is a very common reporting structure in Croatian.

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

So:

• kapetan can mean captain, a captain, or the captain
• otok can mean island, an island, or the island

The exact meaning depends on context.

In this sentence, English naturally uses the captain and the island, but Croatian does not need separate words for that.

Yes, Croatian word order is fairly flexible, though some orders are more natural than others.

This sentence:

• Kapetan kaže da ćemo stići na otok prije mraka.

Could also be rearranged a bit, for example:

• Kapetan kaže da ćemo prije mraka stići na otok.

Both are understandable, but the original sentence sounds very natural and neutral.

The main thing to keep stable is the core grammar:

• da ćemo stići
• na otok
• prije mraka

Yes, doći could also work in many contexts, but stići is very natural here.

Both can relate to arriving, but stići often emphasizes:

• reaching a destination
• sometimes also making it in time

That fits well with:

• prije mraka = before dark

So stići feels especially suitable in this sentence:

• we will make it / arrive before dark

The hardest part for many English speakers is ć.

• ć is a soft sound, somewhat like a softer English ch
• č is a harder sound, more like ordinary English ch

Rough English-style approximations:

• ćemo ≈ CHEH-mo, but with a softer ch
• stići ≈ STEE-chee, again with a softer ch

Also:

• čemo and ćemo are not the same sound in careful Croatian speech

If you want to sound better quickly, aiming for a soft ch is a good start.