Interfon ne radi, pa nam gosti moraju telefonirati kad dođu.

Because raditi here means to work / to function, and in the present tense it is negated with ne:

• radi = it works
• ne radi = it does not work

You do not use nije here, because nije is the negative of je (is), not of radi. So:

• Interfon ne radi = The intercom isn’t working / doesn’t work

This is a very common Croatian pattern with machines and devices:

• Mobitel ne radi. = The phone isn’t working.
• Lift ne radi. = The elevator isn’t working.

Here pa means something like so, therefore, or as a result.

The sentence has this logic:

• Interfon ne radi = The intercom doesn’t work
• pa nam gosti moraju telefonirati = so our guests have to call us

So pa connects the cause and the consequence.

In everyday Croatian, pa is extremely common and can have slightly different shades depending on context, but here it is basically a simple conversational so.

Nam is the dative form of mi (we/us). It means to us.

So:

• telefonirati komu? = to phone whom?
• telefonirati nam = to phone us

That is why Croatian uses nam here, not an accusative form.

The full idea is:

• gosti moraju telefonirati nam = guests have to call us

In normal word order, the short pronoun nam usually comes earlier:

• gosti nam moraju telefonirati

So nam is a clitic pronoun in the dative case.

Because the verb telefonirati normally takes the dative, not the accusative.

So Croatian thinks of this as:

• telefonirati komu = to phone someone

Examples:

• Telefoniram mami. = I’m calling my mum.
• Možeš li nam telefonirati? = Can you call us?

You would not normally say telefonirati nas in standard usage here.

This is a common thing English speakers need to get used to: Croatian verbs often require a different case than the equivalent English verb.

Gosti is the nominative plural of gost (guest).

It is the subject of moraju telefonirati, so nominative is exactly what you want:

• singular: gost
• plural: gosti

Here it means the guests or our guests, depending on context. Croatian often leaves out possessives when they are obvious from the situation.

Moraju is the 3rd person plural present tense of morati, meaning must / have to.

Because the subject is gosti (guests, plural), the verb must also be plural:

• gost mora = the guest must
• gosti moraju = the guests must

So:

• gosti moraju telefonirati = the guests have to call

This is a very common structure in Croatian:

• morati + infinitive

Examples:

• Moram ići. = I have to go.
• Moramo čekati. = We have to wait.
• Gosti moraju telefonirati. = The guests have to call.

In kad dođu, Croatian uses a present-tense form of a perfective verb to refer to a future completed action: when they arrive.

The verb is:

• doći = to come / to arrive

Its 3rd person plural present form is:

• dođu = they come / they arrive

But with kad, it often means when they arrive in the future.

So:

• kad dođu = when they arrive

This is very natural in Croatian. English uses a present form too in clauses with when:

• when they arrive, not when they will arrive

Croatian is doing something similar, but with the perfective verb doći.

This is an aspect question.

• doći is perfective: it focuses on arriving as a completed event
• dolaziti is imperfective: it focuses on the process, repeated coming, or ongoing action

In this sentence, the important idea is the moment the guests arrive, so doći is the natural choice:

• kad dođu = when they arrive

If you used dolaze, it would sound more like when they are coming / when they come regularly, which does not fit as well here.

So the sentence uses perfective aspect because the phone call happens at the point of arrival.

Because short unstressed pronouns like nam usually go near the beginning of the clause. Croatian has a special tendency for these short words, often called clitics, to appear in an early position.

So this sounds natural:

• pa nam gosti moraju telefonirati

This is less natural:

• pa gosti moraju nam telefonirati

Croatian clitic placement is a big topic, but for a learner, a useful rule is:

• short pronouns like mi, ti, mu, joj, nam, vam, im, ga, je, se often come early in the sentence or clause

Yes, but the structure would change.

• telefonirati komu = to phone someone
• nazvati koga = to call someone

So you could say:

• Interfon ne radi, pa nas gosti moraju nazvati kad dođu.

Notice the pronoun changes:

• telefonirati nam → dative
• nazvati nas → accusative

Both sentences are natural, but they use different verbs with different case patterns.

Yes, basically.

• kad = shorter, very common in everyday speech
• kada = longer, sometimes a little more formal or emphatic

In this sentence, kad dođu is completely normal and natural.

You could also say:

• Interfon ne radi, pa nam gosti moraju telefonirati kada dođu.

The meaning stays the same.

Because Croatian has no articles like a or the.

So nouns appear without articles, and the meaning is understood from context:

• Interfon ne radi = The intercom isn’t working
• gosti moraju telefonirati = the guests have to call

Croatian speakers rely on context, word order, and situation instead of articles. This is one of the big differences from English.

No. Telefonirati specifically means to phone / to call by telephone.

The sentence says that because the intercom is broken, the guests must phone us when they arrive.

So:

• interfon ne radi = the intercom doesn’t work
• telefonirati = call by phone

The contrast between those two ideas is exactly the point of the sentence.