Ako interfon opet ne bude radio, stavit ću novi otirač ispred vrata i čekati goste ispred zgrade.

Because after ako when you talk about a future condition, Croatian very often uses future II in the subordinate clause.

So:

• ako ... ne bude radio = if ... is not working / if ... doesn’t work
• main clause: stavit ću... = I will put...

This is a very standard Croatian pattern:

• Ako bude padala kiša, ostat ćemo doma.
• Ako ne bude radio, zvat ću majstora.

Using neće raditi after ako is usually not the normal choice here.

It is future II of raditi.

It is made from:

• budem / budeš / bude / budemo / budete / budu
• plus the l-participle of the main verb

Here:

• bude
• radio

So interfon ne bude radio literally means something like the intercom will not be functioning in that future conditional situation.

Because radio is the masculine singular l-participle, and it agrees with interfon.

• interfon is masculine singular
• so the participle is radio

Compare:

• Ako perilica ne bude radila... → perilica is feminine
• Ako računalo ne bude radilo... → računalo is neuter

So the participle changes to match the subject.

Yes. In Croatian, raditi is very commonly used for machines, devices, systems, and services meaning to work, to function, or to operate.

Examples:

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

So this is completely natural Croatian.

In standard Croatian, when the future auxiliary ću comes after the infinitive, the infinitive usually drops its final -i.

So:

• staviti
  • ću → stavit ću
• čekati
  • ću → čekat ću

You can also put the auxiliary before the infinitive:

• Ja ću staviti novi otirač.

Both are correct, but staviti ću is not standard.

Because one future auxiliary can cover two coordinated infinitives when they share the same subject and time reference.

So:

• stavit ću novi otirač ... i čekati goste ...

means the same as:

• stavit ću novi otirač ... i čekat ću goste ...

The second ću is simply omitted because it is understood.

The preposition ispred takes the genitive.

So:

• ispred vrata
• ispred zgrade

Examples:

• ispred kuće
• ispred škole
• ispred auta

In the sentence:

• zgrade is clearly genitive singular from zgrada
• vrata is also the correct form after ispred, but this noun is a bit special

Because vrata is a plural-only noun in Croatian. It refers to a door even though the form is grammatically plural.

So Croatian says:

• vrata = door
• ispred vrata = in front of the door

This is very common and completely normal. A learner often expects a singular form, but with vrata, Croatian just does not use one in everyday meaning door.

Because čekati takes a direct object in the accusative.

The base form is:

• gosti = guests

But after čekati, you need accusative plural:

• čekati goste = to wait for the guests

So:

• gosti = nominative plural
• goste = accusative plural

This is a very common pattern with masculine animate nouns.

Because the adjective must agree with the noun in gender, number, and case.

• otirač is masculine singular
• here it is in the accusative singular
• for masculine inanimate nouns, accusative singular looks like nominative singular

So:

• novi otirač

Compare:

• nova zgrada
• novo računalo
• novi stol

Opet means again.

It suggests that this is not the first time the intercom has had this problem. So the feeling is:

• if the intercom doesn’t work again
• or if the intercom is not working again

It often carries a slight sense of repetition or annoyance, depending on context.

Yes. Croatian often leaves out subject pronouns when they are already clear from the verb form.

So:

• stavit ću already means I will put
• there is no need to say ja unless you want emphasis

Compare:

• Stavit ću novi otirač. = I’ll put a new doormat.
• Ja ću staviti novi otirač. = I will put a new doormat.
  This can sound more emphatic or contrastive.