Kad se čuje sirena, policajka i vatrogaskinja brzo izlaze iz zgrade.

Here Kad means when.

In Croatian, kad commonly introduces a time clause:

• Kad se čuje sirena... = When the siren is heard / When one hears the siren...

In some contexts, kad can feel close to whenever, depending on the situation:

• Kad padne kiša, ostajemo doma. = When/Whenever it rains, we stay home.

It does not mean if here. For a real conditional if, Croatian usually uses ako.

Se čuje is a very common Croatian structure. It is often used to make the action sound more general, impersonal, or passive-like.

• čuje = he/she/it hears or is heard, depending on context
• se čuje here = is heard / can be heard

So:

• Kad se čuje sirena literally works like When the siren is heard

This is not exactly the same as English passive grammar, but it often translates that way.

Compare:

• Čujem sirenu. = I hear the siren.
• Sirena se čuje. = The siren is heard / can be heard.

The little word se is very common in Croatian and appears in reflexive, impersonal, and passive-like expressions.

Because čuje is a conjugated verb form, while čuti is the infinitive.

• čuti = to hear
• čuje = he/she/it hears or, in this kind of structure, is heard

Croatian needs a finite verb in the clause:

• Kad se čuje sirena... = correct
• Kad se čuti sirena... = not correct

So the verb must agree with the grammatical structure of the clause, and here that gives us čuje.

Sirena is in the nominative singular.

In Kad se čuje sirena, the noun functioning as the thing being heard appears in the nominative in this kind of se construction.

So:

• sirena = nominative singular, siren

If you used a normal active sentence with a person hearing it, then sirena would usually become sirenu in the accusative:

• Policajka čuje sirenu. = The policewoman hears the siren.

That is a useful comparison:

• čuti sirenu = to hear the siren
• sirena se čuje = the siren is heard

Because they refer to female people.

• policajka = policewoman / female police officer
• vatrogaskinja = female firefighter

Croatian often marks gender directly in profession nouns.

Compare:

• policajac = male police officer
• policajka = female police officer
• vatrogasac = male firefighter
• vatrogaskinja = female firefighter

Since the sentence uses two female nouns, the verb later appears in the plural form that matches they:

• izlaze = they go out / are leaving

Because the subject is plural:

• policajka i vatrogaskinja = the policewoman and the firefighter

Two people joined by i (and) make a plural subject, so the verb must be plural:

• izlaze = they go out / leave

Compare:

• Policajka izlazi. = The policewoman goes out.
• Policajka i vatrogaskinja izlaze. = The policewoman and the firefighter go out.

Brzo is an adverb meaning quickly.

It modifies the verb izlaze:

• brzo izlaze = they go out quickly

Croatian adverbs do not change form here, so brzo stays the same.

You could move it around a bit without changing the basic meaning:

• ...brzo izlaze iz zgrade.
• ...izlaze brzo iz zgrade.

But brzo izlaze sounds very natural.

Iz means out of / from, and it requires the genitive case.

So:

• zgrada = nominative singular, building
• iz zgrade = out of the building / from the building

This is a very important pattern:

• iz kuće = out of the house
• iz škole = out of the school
• iz auta = out of the car

So zgrade is the genitive singular form of zgrada.

Because Croatian does not have articles like English a and the.

So a noun like sirena can mean:

• a siren
• the siren

And zgrada can mean:

• a building
• the building

You understand which one is meant from context.

So:

• iz zgrade could be understood as out of the building or out of a building, depending on the situation.

In this sentence, English will often use the because the situation feels specific, but Croatian does not need a separate word for that.

Yes. Croatian often uses the present tense in clauses like this to describe regular events, general situations, or actions that happen when something occurs.

So:

• Kad se čuje sirena, ... izlaze ...
  means something like
• When the siren is heard, ... they quickly leave ...

This can describe:

1. a habitual situation
2. a general rule
3. a vivid present-time narration

English also sometimes does something similar:

• When the alarm goes off, everyone leaves.

So the present tense here is completely natural.

Yes, Croatian word order is fairly flexible, though some versions sound more natural than others.

The given sentence:

• Kad se čuje sirena, policajka i vatrogaskinja brzo izlaze iz zgrade.

Possible variations include:

• Kad se čuje sirena, brzo izlaze iz zgrade policajka i vatrogaskinja.
• Policajka i vatrogaskinja izlaze brzo iz zgrade kad se čuje sirena.

But the original version is very natural because it is clear and neutral:

1. time clause first
2. subject next
3. adverb
4. verb
5. prepositional phrase

Also, se usually stays close to the verb:

• se čuje, not normally far away from it

So yes, the order can change, but the original is a good standard model.

Izlaze comes from izlaziti, which is the imperfective verb.

• izlaziti = to be going out / to go out regularly / repeatedly
• izaći = to go out, to come out (perfective)

In this sentence, the imperfective works well because the whole statement sounds like a general or repeated situation:

• When the siren is heard, they quickly go out of the building.

If you used a perfective form, it would create a different nuance and would not fit this simple present-tense general statement in the same way.

So izlaze is natural because it describes what they do in that situation as a regular response.

It can be understood both ways, which is one reason this Croatian structure is so useful.

Depending on context, it can feel like:

• when the siren is heard
• when one hears the siren
• when the siren can be heard

English often has to choose one phrasing, but Croatian can stay more neutral here.

In this sentence, the important point is not who hears the siren, but simply that the sound of the siren happens and triggers the next action.