Grijanje se opet pokvarilo; mislim da je to novi kvar.

Grijanje means heating or the heating system. In Croatian, it is very common to talk about the system itself this way, not just the physical machine.

So in this sentence, grijanje se opet pokvarilo means the heating system has broken down again.

If you wanted to talk about a specific device, you might use other words depending on context, such as grijalica for a heater.

Here se is part of the verb pokvariti se / kvariti se, which means to break down, to stop working, or to go wrong.

So:

• pokvariti = to spoil something / to break something
• pokvariti se = to break down, to become broken

Examples:

• Pokvario sam sat. = I broke the clock/watch.
• Sat se pokvario. = The clock/watch broke.

In your sentence, the heating is not actively breaking something else; it itself stopped working, so se is needed.

Because grijanje is a neuter singular noun.

In Croatian past tense, the participle agrees with the subject in gender and number:

• masculine singular: pokvario
• feminine singular: pokvarila
• neuter singular: pokvarilo

Since grijanje is neuter, you get:

• Grijanje se pokvarilo.

This is very common with nouns ending in -nje, which are usually neuter.

Opet means again.

So Grijanje se opet pokvarilo means the heating has broken down again — implying this has happened before.

Croatian also has ponovno, which can also mean again, but opet is very common in everyday speech and often sounds a bit more natural in casual conversation.

The semicolon separates two closely related statements:

• Grijanje se opet pokvarilo
• mislim da je to novi kvar

It works a bit like a stronger comma or a softer full stop. It shows that the second part is closely connected to the first.

In everyday writing, many people might also use a comma or a full stop instead:

• Grijanje se opet pokvarilo. Mislim da je to novi kvar.
• Grijanje se opet pokvarilo, mislim da je to novi kvar.

So the semicolon is correct, but it is more a punctuation/style point than a grammar requirement.

In Croatian, da is very often used to introduce a subordinate clause after verbs like think, know, say, hope, and so on.

So:

• Mislim da je to novi kvar. = I think that it is a new fault/problem.

This is the normal Croatian structure.

Compare:

• Znam da je ovdje. = I know that he/she/it is here.
• Kažem da nije istina. = I’m saying that it isn’t true.

So da here corresponds to English that.

Kvar is a noun meaning fault, breakdown, or malfunction.

Pokvarilo se is a verb phrase meaning it broke down or it stopped working.

So the sentence uses both:

• Grijanje se opet pokvarilo = the heating broke down again
• to je novi kvar = that is a new fault/breakdown

A useful way to think of it is:

• kvar = the problem itself
• pokvariti se = the event of becoming broken

Because kvar is a masculine singular noun, so the adjective must agree with it:

• masculine singular: novi kvar
• feminine singular: nova stvar
• neuter singular: novo grijanje

Croatian adjectives change to match the noun in gender, number, and case.

So:

• kvar = masculine
• therefore novi is the correct form

To means that or it here. It refers back to the situation just mentioned: the heating breaking down again.

So the speaker is saying that this new incident is probably a separate or new malfunction, not just exactly the same issue as before.

Croatian often uses to in this broad way to refer to an entire situation or fact, not just a single noun.

Yes. Mislim da je to kvar is grammatically correct and means I think that’s a fault / a breakdown.

But novi adds an important idea: the speaker thinks this is a new fault, not simply the old one continuing.

So:

• to je kvar = it is a fault/problem
• to je novi kvar = it is a new fault/problem

Because Croatian usually drops subject pronouns when they are not needed.

The verb form itself already tells you the subject:

• mislim = I think
• misliš = you think
• misli = he/she thinks
• mislimo = we think

So ja mislim is possible, but ja is usually only added for emphasis, contrast, or clarity.

For example:

• Mislim da je to novi kvar. = I think it’s a new fault.
• Ja mislim da je to novi kvar, a on ne misli tako. = I think it’s a new fault, but he doesn’t think so.

Pokvariti se is perfective. It refers to the event of breaking down as a completed happening.

That fits this sentence well, because the speaker is talking about a specific result:

• the heating has broken down again

The imperfective partner is kvariti se, which is more like to be breaking down, to keep malfunctioning, or to break repeatedly / habitually, depending on context.

Compare:

• Grijanje se pokvarilo. = The heating broke down.
• Grijanje se stalno kvari. = The heating keeps breaking down.

This perfective/imperfective contrast is very important in Croatian verbs.

Yes, Croatian word order is fairly flexible, though not completely free. The original sentence is natural and neutral:

• Grijanje se opet pokvarilo; mislim da je to novi kvar.

But you could also hear variations such as:

• Opet se grijanje pokvarilo.
• Mislim da je to opet novi kvar.
  (this changes the emphasis slightly)
• To je, mislim, novi kvar.

Different word orders can shift emphasis, rhythm, or style. The original version sounds very normal and straightforward.