Čula se sirena, pa je policajac odmah pogledao prema mostu.

Both are possible, but they sound slightly different in emphasis and flow.

• Čula se sirena is a very natural way to introduce a sound event: A siren was heard / One could hear a siren.
• Sirena se čula puts more focus on sirena itself, as the topic.

Croatian often allows flexible word order, and the first version sounds especially natural in storytelling because it presents the event first.

No. Here se is not literal reflexive oneself.

In čula se sirena, se helps create an impersonal or middle/passive-like meaning. A natural English equivalent is:

• A siren was heard
• One could hear a siren

So se is part of a common Croatian structure used when the focus is on what was perceived, not on who perceived it.

Because the past participle agrees with sirena, which is a feminine singular noun.

• sirena = feminine singular
• therefore: čula

Compare:

• Čuo se zvuk. = A sound was heard. (zvuk is masculine)
• Čula se sirena. = A siren was heard. (sirena is feminine)

Because je is a clitic, and Croatian clitics usually go in the second position of their clause.

So in:

• pa je policajac odmah pogledao...

the conjunction pa counts as the first element, and je comes right after it.

This second-position rule is very important in Croatian word order.

Croatian past tense is usually formed with:

• a form of biti (sam, si, je, smo, ste, su)
• plus the past participle

So:

• je pogledao = looked
• je čula would normally be expected too, but in čula se sirena the structure is different and more impersonal/passive-like.

For policajac, the normal past construction is:

• policajac je pogledao

Because policajac is masculine singular, the participle is pogledao.

Pa here means something like:

• so
• then
• and so

It links the two events in sequence:

1. Čula se sirena
2. pa je policajac odmah pogledao prema mostu

So it shows that the second action follows naturally from the first one.

Because these mean different things.

• pogledao most = looked at the bridge
• pogledao prema mostu = looked toward the bridge / in the direction of the bridge

The preposition prema emphasizes direction, not direct visual focus on the object itself.

So here the policeman turned his gaze toward the bridge.

Because prema requires the dative case.

• nominative: most
• dative: mostu

So:

• prema mostu = toward the bridge

This is something learners need to memorize with prepositions: Croatian prepositions often control a specific case.

Policajac is in the nominative singular because it is the subject of the second clause:

• pa je policajac odmah pogledao prema mostu

It is the one doing the action of looking.

Yes, Croatian word order is flexible, so odmah can move, but the meaning emphasis changes slightly.

The original:

• pa je policajac odmah pogledao prema mostu

is very natural and means the policeman immediately looked toward the bridge.

You could also hear:

• pa je odmah policajac pogledao prema mostu
• pa je policajac pogledao odmah prema mostu

But the original sounds the most neutral and natural.

Pogledao is perfective.

That means it describes a single completed action: he glanced/looked.

If you used the imperfective verb gledati, it would suggest more of an ongoing action:

• gledao prema mostu = was looking toward the bridge
• pogledao prema mostu = looked toward the bridge

In this sentence, the policeman reacts quickly to the siren, so the perfective verb fits well.

Grammatically literal thinking may suggest that, but that is not how this structure works in Croatian.

In real usage, čuti se often means:

• to be heard
• to be audible
• to be heard from somewhere

So čula se sirena is a standard Croatian way to say that the sound of a siren was heard.

It sounds quite neutral and natural, especially in narration.

• Čula se sirena is common in storytelling and description.
• pa je policajac odmah pogledao prema mostu is also standard narrative Croatian.

So this is the kind of sentence you could see in a story, news-style narration, or a descriptive passage.