Kad nema vjetra, djeca voze romobil pokraj velikog drveta.

Here kad means when. It introduces a time clause: Kad nema vjetra = When there is no wind.

Yes, kad and kada are basically the same here. Kad is the shorter, very common form; kada can sound a bit fuller or more formal.

Because Kad nema vjetra is a subordinate clause placed before the main clause.

• subordinate clause: Kad nema vjetra
• main clause: djeca voze romobil pokraj velikog drveta

In Croatian, when a clause like this comes first, it is separated by a comma.

Because nema in the sense of there is no / there isn’t normally takes the genitive case.

So:

• nominative: vjetar = wind
• genitive: vjetra

That is why Croatian says nema vjetra, not nema vjetar.

Yes. Nema is the negative form connected with ima.

Very often:

• ima = there is / there are
• nema = there is no / there are no

So in this sentence, nema does not mean doesn’t have in a personal sense. It is being used existentially: there is no wind.

Because djeca means children, so it takes a plural verb.

Even though djeca may look unusual to an English speaker, it behaves as a plural subject here:

• djeca voze = the children ride
• not djeca vozi

So voze is the correct present-tense plural form.

It comes from voziti. This verb can mean different things depending on context, including drive, ride, or transport.

With romobil, the natural English meaning is ride:

• voziti romobil = to ride a scooter

So even though the basic Croatian verb is broad, the object tells you how to translate it naturally.

Romobil is the direct object, so it is in the accusative singular.

But romobil is a masculine inanimate noun, and for many masculine inanimate nouns:

• nominative singular = accusative singular

So:

• nominative: romobil
• accusative: romobil

That is why the form does not change.

Because the subject and the object do not have to match in number.

• djeca = plural subject
• romobil = singular object

This can mean the children are riding one scooter, or the sentence may be speaking more generally. If you wanted to say scooters, you would use romobile.

It is genitive singular.

The reason is the preposition pokraj, which means beside / next to / by. Pokraj takes the genitive case, so both the adjective and the noun must be in genitive:

• velikog = genitive singular
• drveta = genitive singular

Because adjectives in Croatian must agree with the noun in:

• gender
• number
• case

The noun is drvo (tree), which is neuter singular. After pokraj, it has to be in the genitive singular, so the adjective velik changes to velikog to match it.

Drvo is the nominative singular form. After pokraj, you need the genitive singular, which is drveta.

So the pattern is:

• nominative: drvo
• genitive: drveta

This is just part of the noun’s declension.

You usually know from context. Croatian has no articles, so nouns do not automatically show a/an or the.

That means:

• romobil can be a scooter or the scooter
• velikog drveta can be a big tree or the big tree

English has to choose an article, but Croatian often leaves that to context.

Croatian word order is fairly flexible. This sentence has a very natural, neutral order:

• time clause first: Kad nema vjetra
• main clause after that: djeca voze romobil pokraj velikog drveta

You can move parts around for emphasis, but the case endings do a lot of the grammatical work, so Croatian is less rigid than English about word order.