Zbog hladnoće djeca ostaju unutra i ne idu u park.

Because zbog always takes the genitive case in Croatian.

The base form is hladnoća (cold, as a noun), but after zbog you need the genitive singular:

• nominative: hladnoća
• genitive: hladnoće

So:

• zbog hladnoće = because of the cold

This is a very common pattern, and it is worth memorizing that zbog + genitive is fixed.

Hladnoće is the genitive singular of hladnoća.

Hladnoća is a feminine noun meaning cold in the abstract sense, like cold weather or coldness. It is not the adjective cold.

A useful contrast:

• hladno = cold / it is cold (adjective or adverb-like use)
• hladnoća = coldness, the cold (noun)

So this sentence uses the noun form because Croatian says literally because of the cold.

Although djeca may look unusual, it means children and it takes plural agreement.

So you get:

• djeca ostaju
• djeca idu

not singular forms.

This is one of those important Croatian nouns you simply learn as a plural meaning, even though its shape may not look like a typical plural to an English speaker.

A few forms to know:

• nominative: djeca = children
• genitive: djece = of children

So in this sentence, plural verbs are completely normal.

Ostaju is the present tense of the imperfective verb ostajati, while ostanu comes from the perfective verb ostati.

In a sentence like this, Croatian normally prefers the imperfective because it describes what the children do or are doing as a general/current situation:

• djeca ostaju unutra = the children stay inside / are staying inside

If you used ostanu, it would sound more like a completed-result idea and would not fit as naturally here.

This is a common Croatian pattern:

• imperfective = ongoing, repeated, general
• perfective = completed whole event

Unutra is an adverb, meaning inside or indoors.

So:

• ostaju unutra = they stay inside / indoors

Because it is an adverb, it does not need a preposition here.

Compare:

• unutra = inside, indoors
• vani = outside

You could also say something more specific, such as:

• ostaju u kući = they stay in the house/home

But unutra is broader and simply means they remain indoors.

Because Croatian uses different cases depending on whether there is movement toward a place or location in a place.

With u:

• u + accusative = movement into/to
• u + locative = being in

So:

• idu u park = they go to the park
• su u parku = they are in the park

In this sentence, the verb idu involves movement, so park must be in the accusative, which here is park.

In Croatian, ne is usually written separately from the verb:

• ne idu = they do not go
• ne ostaju = they do not stay

So this is the normal spelling.

English speakers sometimes expect a single combined form, but in Croatian the negative particle is usually separate. There are a few common exceptions elsewhere in the language, but with ići here, the correct form is definitely ne idu.

Croatian word order is fairly flexible, but putting Zbog hladnoće first is very natural because it gives the reason/background before the main statement.

So the sentence is structured roughly like:

• Because of the cold, the children stay inside and do not go to the park.

This sounds smooth and neutral.

You could rearrange parts of the sentence for emphasis, but the given order is probably the most natural in everyday use.

Because i simply means and. Croatian does not need a different conjunction just because the next clause is negative.

So:

• ostaju unutra i ne idu u park = they stay inside and do not go to the park

That is perfectly normal.

English does the same thing:

• They stay inside and don’t go to the park.

Yes, Croatian often drops subject pronouns, but here djeca is a full noun, not just a pronoun, and it is included because we want to say explicitly who is staying inside and not going to the park.

If the subject were already clear from context, Croatian might omit a pronoun like ona (they), but keeping djeca makes the sentence complete and clear on its own.

So this sentence is fully natural as written.