Ako me opet zaboli grlo, ostat ću kod kuće.

With boljeti / zaboljeti, Croatian usually treats the person who feels pain as an object, not as the possessor of the body part.

So:

• me = me in the accusative case
• grlo = throat

Literally, the structure is closer to If the throat hurts me again... than to If my throat hurts again...

That is why Croatian does not need a separate word for my here. The relationship is understood naturally.

A very common pattern is:

• Boli me glava. = My head hurts.
• Boli me ruka. = My arm hurts.
• Zabolio me zub. = My tooth started hurting.

So me is exactly what you expect with this verb pattern.

The subject is grlo.

In other words:

• grlo is the thing that hurts
• me is the person affected by the pain

So grammatically, grlo is doing the verb:

• grlo zaboli = the throat starts to hurt
• me zaboli grlo = my throat starts to hurt / more literally the throat starts hurting me

This is different from English, where learners often think of I as the main participant. In Croatian, with this kind of expression, the body part is often the grammatical subject.

Because zaboli and boli do not mean exactly the same thing.

• boli is imperfective: it describes ongoing pain or a general state
• zaboli is perfective: it often means starts to hurt or hurts suddenly / as a single event

So:

• Ako me opet boli grlo... = If my throat is hurting again...
• Ako me opet zaboli grlo... = If my throat starts hurting again...

In your sentence, zaboli fits well because the speaker is talking about a possible future event that may happen again.

This is very normal in Croatian.

After ako meaning if, Croatian usually uses the present tense, even when the meaning is future.

So Croatian says:

• Ako me opet zaboli grlo, ostat ću kod kuće.

not normally:

• Ako će me opet zaboljeti grlo...

This is similar to English in a way. English also says:

• If it rains tomorrow, I’ll stay home

not:

• If it will rain tomorrow...

So the present form zaboli is the normal choice after ako, even though the whole situation refers to the future.

Croatian word order is fairly flexible, but this version is natural and standard.

A few things are happening:

1. Ako introduces the conditional clause.
2. me is a clitic, and clitics usually come very early in the clause, often in second position.
3. opet naturally sits before the verb here.
4. grlo comes later as the subject.

So Ako me opet zaboli grlo sounds very normal.

You can sometimes move things around for emphasis, but not every version sounds equally natural. For example, Croatian is less free with clitics than English learners often expect. The placement of me is especially important.

Because when the future auxiliary ću comes after the infinitive, the infinitive normally loses its final -i in standard Croatian.

So:

• ostati
  • ću becomes ostat ću
• raditi
  • ću becomes radit ću
• pisati
  • ću becomes pisat ću

That is why ostati ću is not the standard form.

Yes. Both are correct.

You can say:

• Ostat ću kod kuće.
• Ću ostati kod kuće.

However, in normal independent sentences, the auxiliary usually does not stand first by itself, so in practice you would more naturally say:

• Ja ću ostati kod kuće.
• Ostat ću kod kuće.

The version in your sentence, ostat ću, is very common and natural.

Kod kuće means at home.

Literally:

• kod often means at or by
• kuće is the genitive form of kuća

But as a fixed expression, kod kuće simply means at home.

This is different from u kući, which means in the house in a more physical, literal sense.

Compare:

• Ostat ću kod kuće. = I’ll stay home.
• Ostat ću u kući. = I’ll stay in the house.

The second one sounds more physical or contrastive, as if the important point is being inside the building.

Because the sentence begins with a subordinate clause:

• Ako me opet zaboli grlo = conditional clause
• ostat ću kod kuće = main clause

In Croatian, when a subordinate clause comes before the main clause, it is normally separated by a comma.

So the comma is standard punctuation here.

Yes. Opet and ponovno can both mean again.

So you could say:

• Ako me opet zaboli grlo...
• Ako me ponovno zaboli grlo...

The difference is mostly one of style:

• opet is very common and everyday
• ponovno can sound a bit more formal or neutral, depending on context

In this sentence, opet sounds completely natural.