Djeca se još nisu navikla na novi vrtić.

Because djeca is the irregular plural of dijete (child). Even though -a often looks like a singular feminine ending, djeca is definitely plural.

This is one of those forms you simply have to learn as a whole:

• dijete = child
• djeca = children

Because djeca takes neuter plural agreement in Croatian.

So with djeca, adjectives and past participles often use the neuter plural form:

• djeca su došla
• djeca se nisu navikla

If the subject were a masculine plural noun like dječaci (boys), then you would use navikli.

Because the verb is naviknuti se na, which means to get used to.

Without se, naviknuti usually means to accustom someone/something to something.

Compare:

• Djeca su se navikla na vrtić. = The children got used to kindergarten.
• Roditelji su naviknuli djecu na vrtić. = The parents got the children used to kindergarten.

So in this sentence, se is necessary.

This is the Croatian perfect tense.

It is formed with:

• the auxiliary biti in the present tense
• plus the l-participle

Here:

• nisu = they have not / they did not as the negative auxiliary
• navikla = the participle

So nisu navikla means something like have not gotten/got used. In Croatian, this tense can often match either English present perfect or simple past, depending on context.

In a negative sentence like this, još usually gives the sense of yet or still.

So:

• nisu se navikla = they did not / have not gotten used
• još se nisu navikla = they still have not gotten used / they have not gotten used yet

It suggests that the process is ongoing and the expected result has not happened so far.

Because naviknuti se normally goes with na + accusative.

So:

• naviknuti se na nešto = to get used to something

In this sentence, novi vrtić is in the accusative singular.

Because vrtić is a masculine inanimate noun. In Croatian, masculine inanimate nouns often have the same form in the nominative and accusative singular.

So:

• nominative: novi vrtić
• accusative: novi vrtić

That is why the case change is not visible here. If it were masculine animate, you would see a clearer difference, for example:

• novi učitelj → novog učitelja

Because the past-participle stem does not always look exactly like the infinitive stem.

For this verb, the usual participle is:

• navikao = masculine singular
• navikla = feminine singular or neuter plural
• naviklo = neuter singular

So navikla is the expected form here with djeca.

Croatian word order is fairly flexible, but short unstressed words have placement rules.

Here, se is a clitic, so it usually appears very early in the sentence, often after the first stressed word or phrase. That is why Djeca se... sounds natural.

The order in this sentence is neutral and idiomatic:

• Djeca se još nisu navikla na novi vrtić.

You can move parts around for emphasis, but not completely freely.

Because naviknuti se is perfective, and it focuses on reaching the point of being used to something.

So:

• nisu se još navikla = they still have not reached that point

The imperfective navikavati se focuses more on the process itself:

• Djeca se još navikavaju na novi vrtić. = The children are still getting used to the new kindergarten.

Both are possible in real life, but they highlight slightly different things.

Vrtić usually means kindergarten, preschool, or sometimes day-care, depending on the country and context.

In many learner translations, kindergarten is the most natural choice, but the exact institution may vary a bit.

A rough English approximation would be:

• djeca ≈ DYE-tsa
• vrtić ≈ VR-teech

A couple of notes:

• dj is a soft sound, not exactly the same as English j
• ć is softer than English ch
• the r in vrtić is tapped or trilled, not the usual English r