Ako se grijanje opet pokvari, zvat ćemo majstora odmah.

After ako in a real future condition, Croatian usually uses the present tense, not a future form.

So:

• Ako se grijanje opet pokvari = If the heating breaks down again
• zvat ćemo majstora odmah = we will call a repairman immediately

Also, pokvariti se is a perfective verb, and its present-tense form often refers to a future single completed event in sentences like this.

Se is a short reflexive particle, but in many verbs it does not mean a literal reflexive action like wash oneself. In pokvariti se, it is just part of the verb and gives the meaning to break down / to go wrong / to get damaged.

So:

• pokvariti = to spoil or damage something
• pokvariti se = to break down, spoil, go bad

Here grijanje se pokvari means the heating breaks down.

Grijanje is a neuter noun meaning heating or heating system. In everyday use, it often refers to the system in a house or building, not just the abstract idea of warmth.

In this sentence, grijanje means something like:

• the heating
• the heating system

It is the subject of the clause.

Because majstora is the accusative singular form of majstor.

Here majstora is the direct object of zvat ćemo:

• majstor = repairman / handyman / tradesman
• zvati majstora = to call a repairman

Since majstor is a masculine animate noun, its accusative singular usually looks like the genitive singular:

• nominative: majstor
• accusative: majstora

It can mean either, depending on context.

Croatian has no articles, so there is no separate word for a or the. That means:

• zvat ćemo majstora could mean we’ll call a repairman
• or we’ll call the repairman

The situation tells you which meaning is intended.

Both are possible.

Croatian future I can be formed in two common ways:

• zvat ćemo
• ćemo zvati

They both mean we will call.

With infinitives ending in -ti, Croatian often drops the -i before ću, ćeš, će, ćemo, ćete, će:

• zvati
  • ćemo → zvat ćemo

This is very normal and standard.

Because se is a clitic: a short unstressed word that usually goes near the beginning of the clause.

In Croatian, clitics often appear in second position, so after an introductory conjunction like ako, you often get:

• Ako se grijanje opet pokvari...

This word order sounds natural and standard.

This is a real / likely future condition.

The pattern is:

• ako
  • present tense
• main clause in the future

So:

• Ako se grijanje opet pokvari, zvat ćemo majstora odmah.
• If the heating breaks down again, we’ll call a repairman immediately.

This is the normal way to talk about something that may really happen in the future.

Because the sentence has two clauses:

1. Ako se grijanje opet pokvari
2. zvat ćemo majstora odmah

The first is a subordinate if-clause, and it is followed by the main clause. In Croatian, this is normally separated by a comma, just as in English when the if-clause comes first.

Yes, Croatian word order is quite flexible, though some versions sound more natural than others.

For example, these are possible:

• Ako se grijanje opet pokvari, odmah ćemo zvati majstora.
• Zvat ćemo majstora odmah ako se grijanje opet pokvari.

The original sentence is very natural. Changing the order usually changes emphasis, not the basic meaning.

Opet means again.

Yes, you could often use ponovno instead:

• Ako se grijanje opet pokvari...
• Ako se grijanje ponovno pokvari...

Both are correct. In everyday speech, opet is very common and a bit more conversational.

Odmah means immediately / right away and modifies the action zvat ćemo.

So the idea is:

• we will call a repairman immediately

It tells you when the calling will happen.

That sounds unnatural in standard Croatian.

Because se is a clitic, it normally does not go after the full noun subject like that. The standard and natural order is:

• Ako se grijanje opet pokvari

So even though Croatian word order is flexible, clitics like se still follow special placement rules.

Not exactly.

• kvari se comes from kvariti se and is imperfective
• pokvari se comes from pokvariti se and is perfective

Very roughly:

• kvari se = is breaking down / keeps malfunctioning / tends to go wrong
• pokvari se = breaks down, as a single event

In this sentence, pokvari se is used because we mean one future event: if the heating breaks down again.