Kad je vrijeme lijepo, djeca ne odlaze odmah kući nego ostaju još malo u parku.

Kad is the shorter form of kada. Both mean when, and both are standard Croatian.

In this sentence, kad is best understood as when/whenever, because the whole sentence describes a repeated situation, not just one specific event:

• Kad je vrijeme lijepo... = When / Whenever the weather is nice...

So kad here introduces a general condition.

Yes, vrijeme can mean both time and weather. Croatian uses the same word for both, and the context tells you which meaning is intended.

Here, because of lijepo and the overall meaning, it clearly means weather:

• vrijeme je lijepo = the weather is nice

Because vrijeme is a neuter singular noun, the adjective has to agree with it.

So:

• lijep = masculine
• lijepa = feminine
• lijepo = neuter

Since vrijeme is neuter, you get:

• vrijeme je lijepo

Yes, absolutely.

Both are natural:

• Kad je vrijeme lijepo
• Kad je lijepo vrijeme

The difference is small:

• vrijeme je lijepo is a normal predicate structure: the weather is nice
• lijepo vrijeme is a noun phrase: nice weather

So both versions work, with only a slight difference in emphasis.

Because djeca means children, so it agrees as plural.

That is why the sentence has:

• djeca ne odlaze
• djeca ostaju

Both verbs are in the 3rd person plural.

A useful thing to remember:

• dijete = child
• djeca = children

So even if djeca may not look like a regular plural ending to an English speaker, it behaves as plural.

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

• ne idem
• ne rade
• ne odlaze

So ne odlaze is the normal spelling.

There are a few common exceptions where the negative is written together, such as:

• nisam
• nemam
• neću

But with odlaze, it stays separate.

Because odlaziti means to leave / go away, while ići is the more general to go.

Here the idea is not just that the children go home, but that they do not leave for home immediately. So odlaze fits very well because it emphasizes departure from the place where they currently are.

Compare:

• ići = to go
• odlaziti = to leave, go away

Because Croatian normally uses nego in the pattern:

• ne X nego Y = not X but Y / not X, rather Y

That is exactly what happens here:

• djeca ne odlaze odmah kući nego ostaju još malo u parku

So the structure is:

• not going home immediately
• but rather staying a little longer in the park

Ali also means but, but after a direct negation like this, nego is the more appropriate word.

Because kući is a very common fixed form meaning home after verbs of motion.

So Croatian says:

• ići kući = go home
• doći kući = come home
• vratiti se kući = return home

This is similar to English, where we also say go home, not go to home.

So in this sentence:

• odlaze odmah kući = they go home immediately

Because ostaju is imperfective, and that fits a repeated or ongoing situation.

This sentence describes a general habit:

• when the weather is nice, the children stay a little longer

So Croatian uses the imperfective present:

• ostajati → ostaju

The form ostanu is from the perfective verb ostati, and perfective present forms are usually not used for this kind of habitual meaning.

A simple learner-friendly contrast:

• ostaju = they are staying / they stay
• ostanu = they end up staying / they stay on one completed occasion

Literally, još malo means still a little.

In this sentence, the natural English meaning is:

• a little longer
• for a bit longer

So:

• ostaju još malo = they stay a little longer

It expresses a small additional amount of time.

Because u can take different cases depending on meaning:

• u + accusative = movement into something
• u + locative = being in a place

Here the children are staying in the park, not moving into it, so Croatian uses the locative:

• u parku = in the park

If the idea were motion into the park, you would use a different form.

It is most naturally understood as a general habit or repeated situation.

Croatian present tense can describe things that happen regularly, not only what is happening at this exact moment.

So this sentence means something like:

• Whenever the weather is nice, the children don’t go home immediately but stay a little longer in the park.

Because Croatian has no articles like a, an, or the.

So Croatian simply says:

• djeca
• vrijeme
• parku

Whether English would translate that as children, the children, weather, the weather, a park, or the park depends on context.

Because Kad je vrijeme lijepo is a subordinate clause at the beginning of the sentence, and Croatian normally separates that kind of clause from the main clause with a comma.

So the structure is:

• Kad je vrijeme lijepo, = subordinate clause
• djeca ne odlaze... = main clause

That comma is standard and expected.