Kad je mrak, djeca ne idu sama u park.

Kad means when.

In this sentence, Kad je mrak introduces a time clause: When it is dark / When it’s dark.

In everyday Croatian, kad is very common for when. You may also see kada, which is a slightly fuller form of the same word. Both are correct.

Croatian often expresses this idea with the noun mrak, which means darkness.

So Kad je mrak is literally something like When there is darkness, but in natural English we say When it’s dark.

A related Croatian expression is mračno je, which also means it is dark. Both are possible, but kad je mrak is a very natural everyday phrase.

Because Kad je mrak is a subordinate clause placed before the main clause.

So the structure is:

Kad je mrak, = When it’s dark,
djeca ne idu sama u park. = children do not go to the park alone.

This is similar to English, where a comma is also often used after an introductory when clause.

Djeca is an irregular plural form of dijete (child).

So:

• dijete = child
• djeca = children

It is very common, but you just have to learn it as an irregular form.

Even though it does not end like many regular plurals, it still behaves as a plural in the sentence, which is why the verb is idu (they go) and not a singular form.

Idu is the 3rd person plural present tense of ići, which means to go.

So:

• idem = I go
• ideš = you go
• ide = he/she/it goes
• idemo = we go
• idete = you go
• idu = they go

Here it matches djeca (children), so djeca idu = children go.

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

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

So djeca ne idu means the children do not go.

This is normal Croatian verb negation. A learner often notices that English uses do not go, while Croatian simply adds ne before the main verb.

Sama means alone or by themselves here, and it agrees with djeca.

With djeca, the agreeing form is sama. So:

• dijete ide samo = the child goes alone
• djeca idu sama = the children go alone

In this sentence, sama describes how the children go: they do not go alone.

Because Croatian uses different cases depending on whether you mean:

• movement toward/into a place
• location in a place

Here, the children are going to the park, so Croatian uses u + accusative:

• u park = to the park / into the park

If you meant in the park as a location, you would use u parku:

• Djeca su u parku. = The children are in the park.

So:

• ići u park = to go to the park
• biti u parku = to be in the park

Because Croatian does not have articles like English a and the.

So park can mean a park or the park, depending on context.

That means u park can be understood as:

• to a park
• to the park

In this kind of sentence, English often chooses the park, but Croatian does not need a separate word for that.

Yes. Croatian word order is more flexible than English word order.

The given sentence is very natural:

Kad je mrak, djeca ne idu sama u park.

But you could also say:

Djeca ne idu sama u park kad je mrak.

That version means essentially the same thing. The difference is mostly in emphasis and information flow. Putting Kad je mrak first sets the time frame immediately: When it’s dark...

It is most naturally understood as a general statement or habitual fact:

When it’s dark, children don’t go to the park alone.

Croatian present tense often works this way, just like English present tense in general truths.

So this sentence does not usually mean that at this exact moment the children are not going. It sounds more like a rule, habit, or typical situation.

Yes, you could.

• Kad je mrak = when it is dark
• Kad je mračno = when it is dark

Both are understandable and natural.
Kad je mrak feels a bit more like when darkness has fallen, while kad je mračno uses the adjective mračno (dark).

In many situations, they are very close in meaning.