Learn Hardcore Croatian: On želi snimati djecu dok trče kroz šumu. - He wants to film the children while they run through the forest.

Djeca is nominative plural (used for the subject):
- Djeca trče. – The children are running.
When djeca is the direct object (the thing being filmed), you need the accusative:
- Snimam djecu. – I am filming the children.

So in On želi snimati djecu…, djecu is the direct object of snimati, so it must be in the accusative case.

Why is it snimati and not snimiti?

Croatian has aspect: snimati is imperfective, snimiti is perfective.

snimati = to be filming / to record as an ongoing activity, without focusing on completion.
snimiti = to record once, to complete the recording.

With željeti (“to want”), using the imperfective snimati usually expresses wanting to do the activity in general:

On želi snimati djecu… – He wants to (be) filming the children (as they run).

If you used snimiti:

On želi snimiti djecu… – He wants to record the children (once, successfully, to have it done).

Both are grammatically correct, but they emphasize different things: snimati = the process, snimiti = the result.

Why is trče in the third person plural, and who is the subject of that verb?

Trče is 3rd person plural present of trčati (“to run”).

The subject of trče is implied; it is the same as djecu (“children”). Croatian often leaves pronouns out when context is clear:

Full form (more explicit):
On želi snimati djecu dok ona trče kroz šumu.
Usual, more natural form:
On želi snimati djecu dok trče kroz šumu.

So they (the children) are the ones running, even though oni/one is not written.

Could you say On želi snimati djecu dok oni trče kroz šumu? Is that better?

You can say it, but usually you don’t need the pronoun oni here.

On želi snimati djecu dok trče kroz šumu.
– Default, natural; the verb form trče already shows that “they” (the children) are running.
On želi snimati djecu dok oni trče kroz šumu.
– Grammatically correct, but oni adds emphasis or contrast (e.g. while *they are running, not someone else*).

Most of the time, native speakers would omit oni in this sentence.

Why is the conjunction dok used here, and not kad?

dok means “while” and expresses actions happening at the same time:

On želi snimati djecu dok trče kroz šumu.
– He wants to film the children while they are running.

kad usually means “when” and can be more neutral about time (a point or period):

Snimat će djecu kad budu trčali kroz šumu.
– He will film the children when they run through the forest.

Here the idea is clearly “while they are running”, so dok is the natural choice.

Why is kroz used, and what case does it require?

kroz means “through” and it always takes the accusative case.

kroz šumu – through the forest (accusative singular of šuma)
kroz grad – through the city
kroz prozor – through the window

So after kroz, you must put the noun into the accusative: šumu, not šuma or šumi.

Why is it šumu and not šuma?

Because šuma (“forest”) is the object of the preposition kroz, and kroz requires the accusative.

The declension of šuma (singular) starts:

Nominative: šuma – The forest is big. (Šuma je velika.)
Accusative: šumu – I’m walking through the forest. (Hodam kroz šumu.)

In kroz šumu, šumu is accusative singular, which is the correct form after kroz.

Can you omit On and just say Želi snimati djecu dok trče kroz šumu?

Yes, absolutely. Croatian is a pro‑drop language: you often omit subject pronouns when the verb ending already shows who the subject is.

On želi snimati djecu… – He wants to film the children…
Želi snimati djecu… – (He/She) wants to film the children…

Both are correct. You include On if you want to emphasize “he” or to avoid ambiguity in a longer context where several people have been mentioned.

Why is it želi snimati, not želi da snima?

Both structures exist, but they have different flavors.

Infinitive construction (very common with verbs of wanting/being able):
- On želi snimati djecu… – He wants to film the children…
- Verb 1 (želi) + infinitive (snimati)
“da” + present tense construction:
- On želi da snima djecu… – Literally: He wants that he films the children…
- Verb 1 (želi) + da
  - finite verb (snima)

In modern, especially Croatian-standard, the infinitive after željeti is usually more neutral and natural: želi snimati.
želi da snima is more typical in some styles or in Serbian/Bosnian usage.

Why is trče in the present tense, and not a future form like će trčati?

After dok meaning “while”, Croatian normally uses the present tense to talk about time that is simultaneous with another action, even if the whole situation is in the future or hypothetical.

On želi snimati djecu dok trče kroz šumu.
– He wants to film the children while they (are) running through the forest.

A form like dok će trčati is usually ungrammatical or very unnatural. The present tense after dok already covers “while they will be running” in future contexts.

Is there any difference between snimati djecu dok trče kroz šumu and snimati djecu dok budu trčali kroz šumu?

Yes:

dok trče (present) – standard, most natural:
- On želi snimati djecu dok trče kroz šumu.
  – He wants to film the children while they run.
dok budu trčali (future/perfective nuance) – more formal/marked, often used to stress future or conditions:
- On će ih snimati dok budu trčali kroz šumu.
  – He will film them while they are running through the forest.

In your sentence, dok trče is the normal, everyday choice. dok budu trčali can sound heavier or more formal and is more often used in conditional or carefully structured future contexts.

What exactly does snimati cover – only filming, or also audio recording?

snimati is quite general: it means to record, and it can refer to video, audio, or even taking photos in some contexts.

snimati film – to shoot a film
snimati djecu – to film / record the children
snimati pjesmu – to record a song
snimati emisiju – to record a show

In this sentence, because of djecu and dok trče kroz šumu, the most natural interpretation is “to film them on video while they’re running.”