Vlasnica stana kaže da će doći sutra.

Because stan is in the genitive singular: stana.

After nouns like vlasnik / vlasnica meaning owner, Croatian normally puts the thing owned in the genitive:

• vlasnik kuće = owner of the house
• vlasnica stana = owner of the apartment

So stana means of the apartment.

Vlasnica is a feminine noun, meaning female owner.

The masculine form is vlasnik.

So this sentence specifically tells you that the owner is a woman.

Kaže is 3rd person singular present tense, meaning says.

It comes from the verb reći or kazati in its present-tense system:

• ja kažem = I say
• ti kažeš = you say
• on/ona kaže = he/she says

Here it matches vlasnica, so: The owner says...

Because Croatian often drops subject pronouns when they are already clear from the verb form or context.

In this sentence, vlasnica stana is already the subject, so you do not need ona:

• Vlasnica stana kaže... = The apartment owner says...
• Ona kaže... = She says...

Adding ona is possible, but usually unnecessary unless you want emphasis or contrast.

Da introduces a clause like English that:

• kaže da... = says that...

So:

• Vlasnica stana kaže da će doći sutra.
• The apartment owner says that she will come tomorrow.

In English, that is often omitted. In Croatian, da is very commonly kept.

This is the normal way to make the future tense in Croatian:

• ću, ćeš, će, ćemo, ćete, će
  • infinitive

Here:

• će = will
• doći = come

So će doći = will come

Because this is inside a da-clause, the clitic će comes early in the clause:

• kaže da će doći = says that she will come

Because doći is perfective, and it fits a single completed arrival in the future.

• doći = to come, to arrive
• dolaziti = to be coming / to come repeatedly / habitually

In this sentence, the meaning is a one-time future event, so doći is the natural choice.

Yes. That is also correct and natural.

Both are possible:

• ...da će doći sutra
• ...da će sutra doći

The difference is mostly about word order and emphasis, not basic meaning.

Very roughly:

• doći sutra can leave sutra slightly more prominent at the end
• sutra doći can make the time feel a bit more integrated into the clause

Croatian word order is more flexible than English.

Sutra is an adverb, meaning tomorrow.

It does not change form here. It simply tells you when the action will happen:

• doći sutra = come tomorrow

Yes, sometimes.

A Croatian speaker could also say:

• Vlasnica stana kaže da dolazi sutra.

That can also mean The apartment owner says she is coming tomorrow.

Using the present for a future event is common when the event is planned or expected. But da će doći is a very clear, straightforward future.

Not always exactly.

Literally, vlasnica stana means female owner of the apartment. Depending on context, in English that could be:

• the apartment owner
• the landlady

But grammatically, the Croatian phrase itself just says female owner, not necessarily anything more specific about the legal or rental relationship.

Yes. In English, both are fine:

• The apartment owner says that she will come tomorrow.
• The apartment owner says she’ll come tomorrow.

But in Croatian, da is normally present:

• kaže da će doći sutra

So when learning Croatian, it is useful to think of da as the regular connector after verbs like say, think, know, and hope.