Kad se popnemo na brdo, dječak uvijek prvo traži gdje može voziti romobil.

Yes. Kad and kada both mean when.

• kad is very common in everyday speech
• kada is a bit fuller and can sound slightly more formal or careful

In this sentence, Kad se popnemo na brdo... is completely natural.

Because Croatian often uses the present tense in time clauses with kad.

Here, popnemo is the present form of the perfective verb popeti se. With kad, that often gives the sense of:

• when we get up
• when we have climbed up
• once we climb up

So even though the form is present, the meaning is about a completed action in a repeated or future-like sequence.

Because the verb is popeti se, not just popeti.

Many Croatian verbs are learned together with se. In this case, popeti se means to climb up / to get oneself up onto something. It is not necessarily strongly reflexive in English; se is just part of the standard verb form.

So you should learn it as a unit:

• popeti se = to climb up
• penjati se = to be climbing / to climb repeatedly

Se is a clitic, a short unstressed word. Croatian clitics usually appear very early in the clause.

So in Kad se popnemo na brdo, the order is natural because se is placed near the beginning of the clause, before the main verb.

This is something English speakers often just have to get used to: the position of little words like se, ga, je, mi, si follows Croatian word-order rules, not English ones.

Because Croatian often drops subject pronouns when they are already clear from the verb ending.

The ending -mo in popnemo tells you the subject is we.

So:

• popnemo = we climb up / we get up
• mi popnemo would only be used if you wanted to stress we

This is very normal in Croatian.

Because na can take different cases depending on whether there is movement or location.

Here there is movement toward a destination:

• na brdo = onto/up the hill

If you were talking about being located there, you would use:

• na brdu = on the hill

So the contrast is:

• penjemo se na brdo = we are climbing up the hill
• mi smo na brdu = we are on the hill

It is accusative singular because na is expressing movement toward a place.

The noun brdo is neuter, and in the singular its nominative and accusative forms are the same:

• brdo = nominative
• brdo = accusative

So even though the case changes grammatically, the form does not change visibly.

Because Croatian has no articles.

There is no direct equivalent of English a or the. Croatian usually leaves that information to context, word order, or the situation.

So dječak can mean:

• a boy
• the boy

Depending on context, the listener knows which is meant.

They do different jobs:

• uvijek = always
• prvo = first

Together, uvijek prvo traži means something like:

• he always first looks for...
• the first thing he always looks for is...

So uvijek tells you this happens every time, and prvo tells you it is the first step in what he does.

Here it means looks for / searches for.

The verb tražiti can sometimes mean:

• to look for
• to search for
• to ask for / request

But in this sentence, with gdje može voziti romobil, it clearly means he is trying to find a suitable place.

If the meaning were ask, Croatian would more likely use something like:

• pita gdje može voziti romobil = he asks where he can ride a scooter

Because Croatian can leave that noun understood if it is obvious.

The sentence could be expanded to:

• dječak uvijek prvo traži mjesto gdje može voziti romobil

That means:

• the boy always first looks for a place where he can ride a scooter

In the original sentence, mjesto is simply omitted because the idea is clear from context. This is natural and common.

Because Croatian uses the infinitive after modal verbs such as moći.

So:

• može voziti = he can ride

There is no separate word corresponding to English to here.

This pattern is very common:

• mogu doći = I can come
• može voziti = he can ride
• možemo vidjeti = we can see

Romobil means scooter, usually a kick scooter in this kind of context.

It stays as romobil because it is a masculine inanimate noun, and in the singular the accusative is the same as the nominative:

• romobil = nominative
• romobil = accusative

Since it is the direct object of voziti, it is accusative here, but the form does not visibly change.