Djeca trče niz stepenice.

Djeca is the irregular plural of dijete, meaning child.

So the pattern is:

• dijete = child
• djeca = children

This is just one of those forms you need to learn as a pair, because it is not made with the most common plural endings. Even though djeca may not look obviously plural to an English speaker, it behaves as a plural in the sentence, which is why the verb is trče.

Because the subject djeca is plural, the verb has to be in the 3rd person plural form.

The infinitive is trčati = to run.

Present tense:

• ja trčim = I run / am running
• ti trčiš = you run / are running
• on/ona/ono trči = he/she/it runs / is running
• mi trčimo = we run / are running
• vi trčite = you run / are running
• oni/one/ona trče = they run / are running

So:

• Djeca trče = the children run / are running

It can mean either, depending on context.

Croatian does not usually make the same simple-present vs. present-continuous distinction that English does. So one present-tense form often covers both meanings:

• Djeca trče can mean The children run
• or The children are running

In a sentence like Djeca trče niz stepenice, the most natural English translation is usually The children are running down the stairs, because it sounds like an action happening right now.

Here niz means down, especially down along something.

So:

• niz stepenice = down the stairs
• niz ulicu = down the street
• niz rijeku = down the river

It is a preposition used with movement in a downward direction.

A useful opposite is:

• uz stepenice = up the stairs

Stepenice is the noun stairs / steps.

After the preposition niz, Croatian uses the accusative case, so stepenice here is accusative plural.

For this noun, the nominative plural and accusative plural look the same:

• nominative plural: stepenice
• accusative plural: stepenice

So the form does not change visibly, even though the case has changed.

Also:

• stepenica = one step / stair
• stepenice = stairs / steps

Because Croatian has no articles.

That means there is no separate word for a, an, or the. Croatian usually leaves that meaning to context.

So:

• djeca can mean children or the children
• stepenice can mean stairs or the stairs

In this sentence, English normally uses the because the situation sounds specific: The children are running down the stairs.

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

The sentence Djeca trče niz stepenice is a neutral, natural order:

• Djeca = subject
• trče = verb
• niz stepenice = direction phrase

But other orders are possible if you want different emphasis, for example:

• Niz stepenice trče djeca

That might emphasize down the stairs first, or sound more literary or contrastive depending on context.

For a learner, the original order is the best basic version to use.

A rough English-friendly guide is:

• Djeca ≈ DYE-tsa
• trče ≈ TR-cheh
• niz ≈ neez
• stepenice ≈ steh-peh-NEE-tseh

A few sound notes:

• č sounds like ch in church
• c sounds like ts in cats
• z sounds like z in zoo
• in trče, the r is pronounced very tightly, with no clear vowel between t and r, which can feel unusual for English speakers

It is a very natural way to say down the stairs, but Croatian can express similar ideas in other ways depending on nuance.

For example:

• niz stepenice = down the stairs
• uz stepenice = up the stairs
• po stepenicama = on the stairs / along the stairs, not necessarily downward

So if you specifically want the idea of downward movement, niz stepenice is the right choice.