Kad je interfon nedostupan, koristimo staro zvono na vratima.

Kad means when in this sentence.

Yes, you can also say kada:

• Kad je interfon nedostupan, koristimo staro zvono na vratima.
• Kada je interfon nedostupan, koristimo staro zvono na vratima.

Both are correct. Kada is a bit more formal or neutral in style, while kad is very common in everyday speech.

Je is the 3rd person singular present tense of biti (to be).

So:

• interfon je nedostupan = the intercom is unavailable

In the full sentence:

• Kad je interfon nedostupan = When the intercom is unavailable

Croatian often uses biti the same way English uses to be with adjectives.

Nedostupan means unavailable, not accessible, or not reachable, depending on context.

Here it means the intercom is not functioning or cannot be used.

It is built from:

• dostupan = available, accessible
• ne- = not

So:

• dostupan = available
• nedostupan = unavailable

Because nedostupan has to agree with interfon.

Interfon is:

• masculine
• singular
• nominative

So the adjective must also be:

• masculine
• singular
• nominative

That gives:

• interfon
  • nedostupan

If the noun changed, the adjective would change too. For example:

• veza je nedostupna — the connection is unavailable
  (veza is feminine)
• zvono je nedostupno — the bell is unavailable
  (zvono is neuter)

Because Croatian often leaves out subject pronouns when they are already clear from the verb ending.

• koristimo = we use

The ending -mo already tells you the subject is we.

So Croatian usually says:

• Koristimo staro zvono na vratima.

and not necessarily:

• Mi koristimo staro zvono na vratima.

You can add mi if you want emphasis, for example:

• Mi koristimo staro zvono na vratima, ne susjedi.
  = We use the old doorbell, not the neighbors.

Koristimo is:

• present tense
• 1st person plural
• from the verb koristiti = to use

So:

• ja koristim = I use
• ti koristiš = you use
• on/ona koristi = he/she uses
• mi koristimo = we use
• vi koristite = you use
• oni/one koriste = they use

In this sentence, koristimo means we use.

Because the adjective has to agree with the noun.

Zvono is:

• neuter
• singular
• nominative/accusative here

So star must take the neuter singular form:

• staro zvono = old bell

Compare:

• stari interfon = old intercom
  (masculine)
• stara vrata = old door
  (formally plural noun)
• staro zvono = old bell
  (neuter)

Yes. Literally, it means bell on the door, but in natural English the best translation is usually doorbell.

So:

• zvono = bell
• zvono na vratima = bell on the door / doorbell

Croatian sometimes uses descriptive phrases where English prefers one compound noun.

This is a very common learner question.

Vrata means door, but grammatically it is a plural-only noun in Croatian. That means it looks plural even when it refers to a single door.

So:

• basic form: vrata = door
• after na in the sense of on/at, you often use the locative
• locative plural of vrata = vratima

That is why you get:

• na vratima = on the door / at the door

This is normal Croatian grammar, even though it feels strange to English speakers.

Na here means on.

So:

• zvono na vratima = the bell on the door

Croatian uses na with the locative case to express location:

• na stolu = on the table
• na zidu = on the wall
• na vratima = on the door

Because the sentence begins with a subordinate clause:

• Kad je interfon nedostupan = When the intercom is unavailable

Then comes the main clause:

• koristimo staro zvono na vratima = we use the old doorbell

In Croatian, a clause introduced by kad/kada is normally separated by a comma from the main clause.

So the structure is:

• Kad je interfon nedostupan, ...
• Kada dođeš, nazovi me.
• Kad pada kiša, ostajemo unutra.

The given word order is natural and neutral:

• Kad je interfon nedostupan, koristimo staro zvono na vratima.

Croatian word order is more flexible than English, but not completely free. You can move things around for emphasis, though some versions sound more marked.

For example:

• Kad je interfon nedostupan, staro zvono na vratima koristimo.

This is grammatical, but it sounds more emphatic, as if contrasting the old bell with something else.

So for learners, the original sentence is the safest and most natural order.

Croatian does not have articles like English a/an/the.

So:

• interfon can mean an intercom or the intercom
• staro zvono can mean an old bell or the old bell

The exact meaning depends on context.

In this sentence, context makes it clear that we mean a specific intercom and a specific old doorbell, so English naturally uses the:

• When the intercom is unavailable, we use the old doorbell.

Yes. Both are correct:

• Kad je interfon nedostupan
• Kad interfon nije dostupan

They mean essentially the same thing:

• nedostupan = unavailable
• nije dostupan = is not available

The version with nedostupan is a bit more compact and slightly more formal-sounding, while nije dostupan is very straightforward and common.