Ako termometar opet pokaže visoku temperaturu, ostat ću kod kuće.

In Croatian, after ako meaning if, it is very common to use the present tense for a future condition.

So:

• Ako termometar opet pokaže... = If the thermometer shows again...
• but in natural English this is usually translated as If the thermometer shows a high temperature again... or If the thermometer shows a high temperature again, I’ll stay home.

Here, pokaže is the present tense of a perfective verb, and that often refers to a future completed event in clauses like this.

A very common pattern is:

• Ako dođeš, nazvat ću te. = If you come, I’ll call you.
• Ako padne kiša, ostat ćemo doma. = If it rains, we’ll stay home.

So even though it looks like present tense, the meaning is future because of the if-clause structure.

This is about aspect, which is very important in Croatian.

• pokazati = perfective
• pokazivati / pokazuje = imperfective

In this sentence, pokaže is used because the speaker means one specific result/event: the thermometer gives a reading and that reading turns out to be high.

So:

• Ako termometar opet pokaže visoku temperaturu...
  = If the thermometer shows a high temperature again...
  one concrete occasion

If you used pokazuje, it would sound more like:

• it is showing it continuously
• it shows it habitually
• the action is ongoing rather than a single outcome

In conditional sentences about a future event, Croatian often prefers the perfective present.

Because that is the normal spelling of the Croatian future tense when the infinitive comes before the auxiliary.

The full infinitive is ostati.
The future auxiliary is ću.

When the infinitive stands before ću, the final -i drops:

• ostati + ću → ostat ću

This is standard:

• raditi + ću → radit ću
• vidjeti + ću → vidjet ću
• ostati + ću → ostat ću

You can also say:

• Ja ću ostati kod kuće.

That version puts ću before the infinitive. Both are grammatical, but ostat ću is very normal here.

Because it is the direct object of the verb pokaže, so it must be in the accusative case.

The basic dictionary form is:

• visoka temperatura = high temperature

But after a transitive verb like pokazati meaning to show, it changes to accusative:

• visoku temperaturu

Why those endings?

• visoka → visoku
• temperatura → temperaturu

That is because temperatura is a feminine noun, and both the adjective and the noun change to match the accusative singular feminine form.

So:

• nominative: visoka temperatura
• accusative: visoku temperaturu

Kod kuće is the normal Croatian expression for at home or home in the sense of staying where you live.

So:

• ostat ću kod kuće = I’ll stay at home

By contrast:

• u kući means in the house

That refers more literally to being inside a building. It does not always mean at home.

Compare:

• Danas ću ostati kod kuće. = Today I’ll stay home.
• On je u kući. = He is in the house.

So in your sentence, kod kuće is the natural idiomatic choice.

Opet means again.

In this sentence:

• Ako termometar opet pokaže visoku temperaturu...
• If the thermometer shows a high temperature again...

Its position is natural because it comes close to the verb it modifies: pokaže.

Croatian word order is flexible, but not completely free.
Opet can sometimes move for emphasis, but the version here is the most neutral and natural.

For example:

• Opet pada kiša. = It’s raining again.
• Ako opet zakasniš, otići ćemo bez tebe. = If you’re late again, we’ll leave without you.

Yes. In standard Croatian spelling, a subordinate clause introduced by ako is separated from the main clause by a comma.

So:

• Ako termometar opet pokaže visoku temperaturu, ostat ću kod kuće.

The first part is the if-clause.
The second part is the main clause.

This is similar to English, where a comma is also usually used when the if-clause comes first:

• If the thermometer shows a high temperature again, I’ll stay at home.

Yes, that is grammatical.

But it has a slightly different feel.

• ostat ću kod kuće = neutral, natural
• ja ću ostati kod kuće = more emphasis on I

You might use ja if you want contrast, for example:

• Ako termometar opet pokaže visoku temperaturu, ja ću ostati kod kuće, a ti idi bez mene.

So in your original sentence, leaving out ja sounds more natural unless you specifically want emphasis.

In context, it often means a fever or an elevated body temperature.

Croatian, like English, can use temperature in a medical sense. So:

• imati temperaturu = to have a temperature / to have a fever
• visoka temperatura = high temperature / fever

Because the subject is termometar, the sentence strongly suggests a medical reading: the thermometer is measuring someone’s body temperature.

So a learner should understand that this is not only about temperature as a number in general, but very naturally about being ill.

Yes, these are tricky for English speakers.

A few key points:

• č is a hard ch sound, roughly like ch in church
• ć is softer and more palatal; many learners hear it as a softer ch
• ću is pronounced roughly like a soft tyoo / chyoo, depending on accent and how precisely you imitate it
• kuće contains ć, so it sounds softer than kuče would

For practical learning:

• ću → listen carefully and imitate native audio
• kuće → think koo-cheh, but with a softer ch

Even if you do not master the difference immediately, Croatians will usually understand you. But it is a real sound distinction, so it is worth practicing.

Yes, to some extent. Croatian has relatively flexible word order, but different orders sound more or less natural or add emphasis.

The given version is very natural:

• Ako termometar opet pokaže visoku temperaturu, ostat ću kod kuće.

Some possible variations:

• Ostat ću kod kuće ako termometar opet pokaže visoku temperaturu.
• Ako termometar pokaže opet visoku temperaturu, ostat ću kod kuće.
  grammatical, but less natural than putting opet before pokaže
• Ako opet termometar pokaže visoku temperaturu, ostat ću kod kuće.
  possible, with slightly different emphasis

So yes, the order can change, but the original sentence is a very good neutral model to learn from.