Treba pomesti kuhinju prije nego što djeca dođu iz škole.

Treba here means it is necessary, it needs to be done, or one should.

In this sentence, it is used impersonally, without naming a specific person who must do the action. So:

• Treba pomesti kuhinju = The kitchen needs to be swept / Someone should sweep the kitchen

This is a very common Croatian structure:

• Treba učiti = One should study
• Treba kupiti kruh = Bread needs to be bought

Because pomesti is the perfective verb, while mesti is imperfective.

• mesti = to sweep, in a general or ongoing sense
• pomesti = to sweep up, to finish sweeping, to complete the action

In this sentence, the idea is that the kitchen should be swept completely before the children arrive, so the perfective verb pomesti is the natural choice.

This is very common in Croatian: when talking about a single completed action, perfective is often used.

Kuhinju is the accusative singular of kuhinja.

It is in the accusative because it is the direct object of pomesti:

• pomesti što? → kuhinju

So:

• nominative: kuhinja = kitchen
• accusative: kuhinju

This is exactly the kind of case change English speakers need to watch for, since English usually does not change the noun form here.

Prije nego što means before or more literally before ... happens.

It introduces a subordinate clause:

• prije nego što djeca dođu iz škole = before the children come home from school

You can think of it as a fixed expression. It is very common when one action must happen earlier than another.

Examples:

• Operi ruke prije nego što jedeš. = Wash your hands before you eat.
• Nazovi me prije nego što kreneš. = Call me before you leave.

After prije nego što, Croatian often uses the present tense form even when English uses a future idea.

So:

• prije nego što djeca dođu literally looks like before the children come
• but in English the meaning may be before the children come / before the children get back

Also, dođu is from the perfective verb doći, and perfective present forms often refer to a future completed event in subordinate clauses.

So although it looks like present tense, the meaning is future-oriented:

• before the children arrive

Because the subject is djeca = children.

Even though djeca can be confusing to learners, it behaves as a plural noun in meaning, and the verb agrees accordingly:

• djeca dođu
• djeca idu
• djeca spavaju

So dođu is the correct plural form.

Djeca means children, so it is plural in meaning.

It is an important word to memorize as a whole, because it does not look like a regular plural formed from dijete.

• dijete = child
• djeca = children

This is an irregular pair, similar to English child / children.

Because the preposition iz takes the genitive case and means out of or from.

So:

• škola = school
• iz škole = from school

This is a standard pattern:

• iz kuće = from the house
• iz grada = from the city
• iz škole = from school

In this sentence, dođu iz škole means come back from school.

Yes, Croatian word order is more flexible than English word order.

The original sentence:

• Treba pomesti kuhinju prije nego što djeca dođu iz škole.

is natural and neutral.

But other orders are also possible, depending on emphasis:

• Kuhinju treba pomesti prije nego što djeca dođu iz škole.
• Prije nego što djeca dođu iz škole, treba pomesti kuhinju.

All of these are grammatical. The difference is mainly in focus and style, not basic meaning.

In standard Croatian, prije nego što is the safest and most natural full form.

You may sometimes hear shorter variants in speech or see related structures, but for learners it is best to use:

• prije nego što + clause

So:

• prije nego što djeca dođu is the recommended form

That will sound correct and natural in standard usage.

No. The sentence does not say who must do it.

That is because treba + infinitive is impersonal here:

• Treba pomesti kuhinju = The kitchen needs to be swept / Someone should sweep the kitchen

If you wanted to say who should do it, you would need a different structure, for example:

• Moram pomesti kuhinju = I have to sweep the kitchen
• Moraš pomesti kuhinju = You have to sweep the kitchen
• Mama treba pomesti kuhinju = Mom should sweep the kitchen

So the original sentence focuses on the necessity of the action, not the person.

Because Croatian infinitives do not use a separate particle like English to.

The infinitive is just the verb form itself:

• pomesti = to sweep
• doći = to come
• učiti = to study

So in treba pomesti, the verb pomesti already means to sweep. There is no extra word needed.

This is normal in Croatian:

• Želim jesti. = I want to eat.
• Moram raditi. = I have to work.
• Treba pomesti kuhinju. = The kitchen needs to be swept.