Ukoliko kartica ostane u bankomatu, pozovite banku.

Ukoliko means if. In this sentence, it introduces a condition.

It is very close in meaning to ako, but ukoliko usually sounds a bit more formal, official, or written. So in everyday speech, many people would simply say:

Ako kartica ostane u bankomatu, pozovite banku.

Both are correct here.

Because kartica is the subject of the clause kartica ostane u bankomatu. The subject is in the nominative case, and kartica is the nominative singular form.

So here:

• kartica = the card, as the thing doing the “remaining/staying”
• banku = the bank, as the direct object of pozovite

Ostane is a form of the verb ostati, meaning to remain, to stay, or in this context something like to be left in.

Here it is used in a conditional clause: if the card remains/is left in the ATM.

A key point is that ostati is a perfective verb. In Serbian, after words like ako or ukoliko, perfective present forms are often used for a future possible event. So ostane does not mean a present ongoing action like is staying right now. It means something more like:

• if the card ends up remaining in the ATM
• if the card gets left in the ATM

This is mainly a question of aspect.

• ostane comes from ostati, which is perfective
• ostaje comes from ostajati, which is imperfective

In this sentence, ostane is the natural choice because the sentence talks about a single possible event: the card gets left in the ATM.

Ostaje would suggest a more repeated, ongoing, or habitual sense, which does not fit as well here.

So:

• ako kartica ostane... = if the card gets left / remains
• kartica ostaje... = the card is staying / keeps remaining (different kind of meaning)

Because u can take different cases depending on meaning:

• u + accusative = movement into
• u + locative = location in

Here the card is already in the ATM, so Serbian uses the locative:

• u bankomatu = in the ATM

Compare:

• Stavio sam karticu u bankomat. = I put the card into the ATM.
• Kartica je ostala u bankomatu. = The card remained in the ATM.

Bankomatu is the locative singular of bankomat.

The noun bankomat is masculine, and in this sentence it follows u with a location meaning, so it appears in the locative:

• nominative: bankomat
• locative: u bankomatu

So u bankomatu literally means in the ATM.

Pozovite is the imperative form used for:

• you plural
• or formal singular you

In warnings, notices, and instructions, Serbian often uses this more formal/polite form.

So this sentence sounds like a standard public instruction, similar to what you would see on a machine, sign, or official notice.

Compare:

• pozovi = call! (informal, to one person you know well)
• pozovite = call! (formal singular or plural)

Because banku is the accusative singular of banka.

The verb pozvati takes a direct object: you call someone/something. In Serbian, that direct object goes in the accusative case.

So:

• nominative: banka = the bank
• accusative: banku = the bank, as the thing being called

That is why the sentence says pozovite banku.

Yes, in normal writing it is expected.

The sentence begins with a subordinate conditional clause:

Ukoliko kartica ostane u bankomatu

Then comes the main clause:

pozovite banku

In Serbian, when this kind of dependent clause comes first, it is normally separated from the main clause by a comma.

So the punctuation here is standard and natural.

Yes, Serbian word order is fairly flexible, although some versions sound more natural than others.

For example, you could also say:

• Pozovite banku ukoliko kartica ostane u bankomatu.
• Ukoliko u bankomatu ostane kartica, pozovite banku.

These still mean essentially the same thing. The original version is a very natural, clear, standard order.

By itself, kartica can mean card in several senses, depending on context. It can be a bank card, SIM card, ID-type card, and so on.

But because the sentence mentions bankomat (ATM), it is clearly understood here as a bank card / debit card / credit card, depending on context.

So the surrounding words tell you what kind of card is meant.

A slightly less formal, more everyday version would often be:

Ako kartica ostane u bankomatu, pozovite banku.

If you were speaking informally to one person, you might also hear:

Ako kartica ostane u bankomatu, pozovi banku.

So the original sentence sounds a bit more like an instruction, notice, or official message, while these alternatives can sound more conversational.