Kad interfon ne radi, gosti ostavljaju mokre cipele na otiraču i zvone na vrata.

Both kad and kada mean when.

• kad is shorter and very common in everyday Croatian
• kada sounds a bit fuller or more formal

In this sentence, both would work. Kad interfon ne radi... is completely natural.

No. Croatian raditi is used both for:

• a person working
• a machine or device functioning

So:

• Ja radim. = I work.
• Interfon radi. = The intercom works.

That makes interfon ne radi the normal way to say the intercom doesn’t work.

In Croatian, ne is the negation particle, and it usually stands before the verb as a separate word:

• ne radi
• ne zvoni
• ne dolaze

A few very common negative forms are written together, such as nisam and neću, but ne radi stays separate.

Because gosti is the nominative plural form, and this noun is the subject of the sentence.

• gosti = guests, as the subject
• goste = guests, as a direct object

Here, the guests are doing the actions:

• gosti ostavljaju...
• gosti zvone...

So gosti is the correct form.

Because Croatian often leaves out possessive words when ownership is obvious from context.

So gosti ostavljaju mokre cipele is naturally understood as:

• the guests leave their wet shoes

If you wanted extra emphasis, you could say svoje mokre cipele, but it is not necessary here.

Because the adjective mokre must agree with cipele in:

• gender
• number
• case

Here:

• cipele is feminine plural
• it is the direct object
• so it is in the accusative plural
• mokre matches that form

Also, in feminine plural, nominative and accusative often look the same, so mokre cipele has the same shape you might also see in a subject phrase.

Because otiraču is the locative singular of otirač, and the phrase tells you the place where the shoes are left: on the doormat.

With na, Croatian often makes this distinction:

• na + locative = location
  na stolu, na zidu, na otiraču
• na + accusative = movement toward a surface/place
  na stol, na zid

With ostaviti, Croatian very often uses a location phrase like na stolu or na otiraču to show where something ends up.

In Croatian, vrata is a plural-only noun. It usually refers to one door as a single unit, even though its form is plural.

So Croatian normally says:

• vrata = door

That is completely standard. This is why other forms also look plural, such as:

• na vratima
• na vrata

Because zvoniti na vrata is a fixed, idiomatic expression meaning:

• ring at the door
• ring the doorbell

So:

• zvoniti na vrata = ring at the door
• kucati na vrata = knock on the door

By contrast, na vratima usually means a literal location:

• Natpis je na vratima. = The sign is on the door.

So in this sentence, na vrata is the natural idiomatic choice after zvone.

Because the sentence describes a general, repeated situation, not one specific event.

Croatian uses the present tense very naturally for this kind of meaning:

• Kad interfon ne radi... = When the intercom doesn’t work...
• gosti ostavljaju... i zvone... = guests leave... and ring...

This is similar to English sentences like:

• When it rains, people stay inside.

So the present tense here expresses a habitual pattern.

You could, but the meaning would change.

Here, ostavljaju and zvone are imperfective, which fits a habitual or repeated situation. The sentence is describing what guests generally do in that circumstance.

Perfective forms such as ostave and pozvone would sound more like completed, one-time actions in each instance, and less like a broad general description.

So for a neutral statement about usual behavior, the imperfective forms are the best choice.

Not completely. Croatian word order is flexible, but this sentence uses a very neutral and natural order:

Kad interfon ne radi, gosti ostavljaju mokre cipele na otiraču i zvone na vrata.

You can move parts around for emphasis, but the meaning stays mostly the same. For example:

• Kad interfon ne radi, na otiraču gosti ostavljaju mokre cipele...

That version emphasizes na otiraču.

Still, some things are fairly stable:

• kad introduces the subordinate clause
• ne normally comes directly before the verb
• subject + verb + object is the most neutral order for the main clause