Ako kvar ne bude riješen do večeri, zvat ćemo majstora zbog grijanja.

Because the sentence is talking about a future condition: if the fault is not resolved by evening.

After ako in Croatian, you usually do not use ordinary future I (će biti) the way English uses will. Instead, Croatian commonly uses a form like bude or a perfective present form.

So:

• Ako kvar ne bude riješen... = natural for a future condition
• Ako kvar nije riješen... = sounds more like a present situation, if the fault is not resolved/isn't resolved
• Ako kvar neće biti riješen... = generally not the normal choice here

So this is a very common Croatian pattern:
Ako + future situation, then main clause in the future.

Bude is a form of biti (to be), here used in a future-oriented subordinate clause.

In this sentence it combines with riješen:

• bude riješen = is resolved / gets resolved in a future-condition sense

So Ako kvar ne bude riješen... literally looks a bit like If the fault not be resolved..., but in real English it is simply:

• If the fault is not resolved...

This is one of those places where Croatian grammar does something different from English even though the meaning is straightforward.

It is a passive participle used in a passive construction.

The base verb is riješiti = to solve / resolve.
From that, Croatian forms riješen = resolved / solved.

So:

• kvar je riješen = the fault is resolved
• kvar ne bude riješen = the fault is not resolved

It behaves a lot like an adjective because it must agree with the noun it describes.

Since kvar is:

• masculine
• singular
• nominative

the form is:

• riješen

If the noun were feminine, you would get riješena; if neuter, riješeno.

Not exactly.

Kvar usually means a fault, malfunction, breakdown, or defect, especially in a technical or mechanical sense.

So for heating, electricity, appliances, pipes, and similar things, kvar is very natural.

Compare:

• problem = a general problem
• kvar = a technical fault or malfunction

In this sentence, kvar suggests that something in the heating system is not working properly.

Because the preposition do takes the genitive case.

The noun is večer = evening.
Its genitive singular form is večeri.

So:

• do večeri = by evening / until evening

This is a standard pattern:

• do jutra = by morning
• do kraja = until the end
• do kuće = up to the house / as far as the house

In this sentence, by evening is the best English match.

Why? Because the idea is that the repair should happen before that deadline. If it does not happen by then, they will call the repairman.

So:

• Ako kvar ne bude riješen do večeri...
  = If the fault is not resolved by evening...

In other contexts, do večeri can sometimes feel closer to until evening, but here it clearly functions as a deadline.

Because ćemo is a clitic-like future auxiliary and normally cannot stand at the very beginning of its clause by itself.

So Croatian often places the infinitive first:

• zvat ćemo

When that happens, the infinitive zvati loses its final -i:

• zvati → zvat ćemo

If another word comes first, then you can have:

• Sutra ćemo zvati majstora.
• Onda ćemo zvati majstora.

So in this sentence, zvat ćemo is the natural neutral order.

Because majstor is a masculine animate noun, and in Croatian the accusative singular of masculine animate nouns looks like the genitive singular.

So:

• nominative: majstor
• accusative: majstora

That is why:

• zvat ćemo majstora = we will call the repairman

Compare:

• masculine animate: vidim čovjeka = I see the man
• masculine inanimate: vidim stol = I see the table

Since a repairman is a person, Croatian uses the animate pattern.

Because zbog takes the genitive case.

The noun is grijanje = heating.
Its genitive singular is grijanja.

So:

• zbog grijanja = because of the heating / about the heating

In this sentence, the most natural English understanding is something like:

• we’ll call a repairman about the heating

Literally, though, zbog means because of / on account of.

Usually yes, or at least that the call is connected to the heating system.

In context, zbog grijanja means they are calling the repairman because of an issue related to the heating.

So even if the literal meaning is because of the heating, the natural interpretation is:

• about the heating
• for the heating problem
• because the heating is not working properly

Croatian often uses this kind of phrasing where English might prefer a slightly more specific wording.

Yes, absolutely. That is a very natural alternative.

Compare:

• Ako kvar ne bude riješen do večeri...
• Ako se kvar ne riješi do večeri...

Both mean essentially the same thing:

• If the fault is not resolved by evening...

The difference is mostly stylistic:

• ne bude riješen = more passive, a bit more formal or impersonal
• se ne riješi = often more direct and common in everyday speech

So a learner should understand both.

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

So nouns appear without articles:

• kvar
• majstora
• grijanja

Whether English needs a or the depends on context:

• kvar could be the fault
• majstora could be a repairman or the repairman
• grijanja could be the heating

Croatian usually expresses definiteness through context, not with articles.

Croatian word order is fairly flexible, but the given version is natural and neutral.

This sentence:

• Ako kvar ne bude riješen do večeri, zvat ćemo majstora zbog grijanja.

sounds normal and clear.

You can move parts around for emphasis, for example:

• Ako kvar ne bude riješen do večeri, majstora ćemo zvati zbog grijanja.
• Majstora ćemo zvati ako kvar ne bude riješen do večeri.

The meaning stays very similar, but the focus changes slightly.

The main thing to watch is the placement of clitics like ćemo, which cannot just go anywhere.