Learn Hardcore Croatian: Djeca biraju film za večeras. - The children are choosing a movie for tonight.

Djeca biraju = The children choose/are choosing
If the subject were singular, the verb would be singular:
Dijete bira film za večeras. = The child chooses a film for tonight.

What exactly is djeca in terms of gender and number?

Djeca means children, and it is grammatically neuter plural.

The singular is dijete (child), which is neuter singular.
So the pair is: dijete (sg.) → djeca (pl.).
Even though djeca looks irregular, you treat it like any other plural subject when choosing the verb form: djeca biraju, djeca idu, djeca spavaju, etc.

How do you pronounce djeca?

In Croatian, dj is usually pronounced as a single sound, similar to the d in during combined with a soft y sound.

Roughly: djeca ≈ [dye-tsa]
dje sounds like dye, and ca like tsa.
So you do not say it as two separate consonants d
- j in English style; it’s one softened consonant.

What is the infinitive of biraju, and how is it conjugated?

The infinitive is birati (to choose). Present tense conjugation:

ja biram – I choose
ti biraš – you choose (singular, informal)
on/ona/ono bira – he/she/it chooses
mi biramo – we choose
vi birate – you choose (plural or polite)
oni/one/ona biraju – they choose

In the sentence, djeca biraju uses the 3rd person plural form.

Why don’t we say something like Djeca su biraju film… to match English are choosing?

Croatian does not use the verb biti (to be) to form a present continuous tense the way English does.

English: They are choosing
Croatian: Oni biraju (one simple present form covers both choose and are choosing, depending on context).
So Djeca su biraju… is incorrect. You just say Djeca biraju film za večeras.

What aspect is birati, and is there a perfective partner?

Birati is an imperfective verb, focusing on the ongoing or repeated action of choosing.
Common perfective partners are:

izabrati – to choose (complete the act of choosing)
odabrati – to select, pick out

For example:

Djeca biraju film. – The children are (in the process of) choosing a film.
Djeca su izabrala film. – The children have (already) chosen a film.

Why is there no word for the or a in Djeca biraju film za večeras?

Croatian has no articles like English a/an or the.

film can mean a film, the film, or simply film depending on context.
The definiteness is understood from the situation or previous mention, not from a separate word.

What case is film, and why does it look like the base form?

Film here is in the accusative singular as the direct object of biraju.
For masculine inanimate nouns like film, the nominative singular and accusative singular have the same form:

Nominative: film (subject) – Film je zanimljiv.
Accusative: film (object) – Gledam film.

So although it looks like the base form, its function in the sentence shows it is accusative.

What does za mean here, and which case does it usually take?

In this sentence, za means for (indicating purpose/time: for tonight).
Normally, za takes the accusative case with nouns and pronouns:

za Marka – for Mark
za mene – for me
za sutra – for tomorrow

With večeras, you don’t see case endings because večeras is an adverb, not a noun.

What kind of word is večeras? Does it change form?

Večeras is an adverb of time meaning roughly this evening / tonight.

It is invariable: it does not change for case, number, or gender.
You can use it alone (without za):
Gledamo film večeras. – We’re watching a film tonight.

What is the difference between za večeras and just večeras?

Both are common, but there is a nuance:

Djeca biraju film večeras.
Focus is more on when they are doing the choosing – they are choosing this evening.
Djeca biraju film za večeras.
Focus is on the purpose/time of the film – they are choosing a film for tonight (to watch tonight).

In many contexts you could use either, and the intended meaning would still be clear.

Can the word order change, for example Za večeras djeca biraju film?

Yes. Croatian word order is relatively flexible, and you can move elements for emphasis. For example:

Djeca biraju film za večeras. – neutral, straightforward.
Za večeras djeca biraju film. – slightly emphasizes for tonight.
Djeca za večeras biraju film. – mild emphasis on tonight as the time for which the film is intended.

All are grammatically correct; the differences are mostly in focus and style.

How would I say the same idea with a singular subject, e.g. The child is choosing a film for tonight?

You change the subject and adjust the verb to singular:

Dijete bira film za večeras.

Here:

dijete – child (neuter singular)
bira – 3rd person singular of birati
The rest of the sentence stays the same.

I’ve seen deca in some places. Why is it djeca here?

Djeca is the standard Croatian form for children.
Deca is the regular form in Serbian (and some regional varieties), but if you are learning Croatian, you should use djeca.
In Croatian materials, djeca will be considered the correct and standard spelling.