Learn Hardcore Croatian: Djeca uče da je važno poštovati pravila. - Children learn that it is important to respect the rules.

Djeca uče = Children learn / are learning.
- učiti is imperfective: it focuses on the process, habit, or general truth.
Djeca nauče would mean Children (eventually) learn / will learn (they reach the result of knowing).
- naučiti is perfective: it focuses on the completed result, not the ongoing process.
Djeca uče se would usually be understood as Children are learning themselves (reflexive), which is wrong here.
- The verb učiti se exists and means to study, but it’s mostly used with what they study:
  - Djeca se uče čitanju. – Children are learning to read.

In this sentence we want the general idea children are (being) taught / are learning something, so Djeca uče is the normal choice.

What exactly does da do in da je važno poštovati pravila?

Da here is a conjunction meaning that, introducing a subordinate clause which is the object of uče.

Structure:

Main clause: Djeca uče – Children learn
Subordinate clause: da je važno poštovati pravila – that it is important to respect rules

So the whole thing is:

Djeca uče [da je važno poštovati pravila].
Children are learning [that it is important to respect rules].

In Croatian you cannot just drop da in this sentence the way English sometimes drops that.

English: Children learn (that) it is important to respect rules.
Croatian: Djeca uče da je važno poštovati pravila.
- Djeca uče je važno… – ungrammatical. You need the da.

Why do we need je in da je važno poštovati pravila? Could we say da važno poštovati pravila?

Je is the present tense of biti (to be) and corresponds to English is in this sentence.

The English pattern is:

it is important to respect rules

Croatian mirrors that:

(To) je važno poštovati pravila. – It is important to respect rules.
Inside the da clause, it becomes:
da je važno poštovati pravila – that it is important to respect rules.

You cannot omit je here in standard Croatian:

da važno poštovati pravila – incorrect.
You must say:
da je važno poštovati pravila.

Why is važno in the neuter form? What is it agreeing with?

Važno is the neuter singular form of the adjective važan / važna / važno (important).

It agrees with an implicit infinitive / clause functioning as the subject, not with a concrete noun like djeca.
The underlying idea is something like:

(Poštovati pravila) je važno. – (To respect rules) is important.

When the subject is:

an infinitive (like poštovati) or
a whole clause (like poštovati pravila)

Croatian usually uses neuter singular for the predicate adjective:

Plivati je zdravo. – Swimming is healthy.
Biti iskren je važno. – Being honest is important.

So važno is neuter singular because it refers to the whole action poštovati pravila, not to djeca.

Why is poštovati in the infinitive? Could we say Djeca uče da je važno poštuju pravila?

Poštovati is the infinitive, corresponding to English to respect. Croatian often uses the infinitive after adjectives like važno, dobro, teško, to express to do something:

Važno je učiti. – It is important to study.
Dobro je piti vodu. – It is good to drink water.
Teško je razumjeti. – It is hard to understand.

So:

da je važno poštovati pravila = that it is important to respect rules

If you said:

Djeca uče da je važno poštuju pravila. – incorrect form; mixing structures
Grammatically you could say:
Djeca uče da je važno da poštuju pravila. – Children are learning that it is important that they respect rules.

But this is heavier and stylistically less natural than the simple infinitive clause važno poštovati pravila. The infinitive is the normal, smooth way here.

Why is pravila in the plural, and what case is it?

Pravilo = rule (singular, neuter)
Pravila = rules (plural, neuter)

Here, pravila is the accusative plural, functioning as the direct object of the verb poštovati (to respect):

poštovati koga/što – to respect whom/what
- poštovati pravila – to respect rules

For neuter nouns, nominative and accusative plural have the same form: pravila.

Using plural sounds natural because the speaker means rules in general, not a single rule:

poštovati pravila – respect rules (rules in general)
poštovati pravilo – respect the rule (one specific rule).

Can the word order change, for example to Djeca uče da je poštovati pravila važno?

This is possible, but less typical and with a slightly different emphasis.

Standard, most neutral word order:

Djeca uče da je važno poštovati pravila.
- Focus: on the importance (važno) of respecting rules.

Alternatives:

Djeca uče da je poštovati pravila važno.
- Grammatically OK. It slightly emphasizes poštovati pravila as the thing that is important.
Djeca uče da poštovati pravila je važno.
- Feels awkward and unnatural; clitic je does not like this position.

Remember Croatian clitic rule: je tends to go to the second position in its clause. In da je važno poštovati pravila, the order da + je + važno + … is exactly what the language prefers.

Why is there no comma before da in Djeca uče da je važno poštovati pravila?

In Croatian, da introducing an object clause after verbs of thinking, speaking, feeling, learning, etc. is usually not preceded by a comma.

Examples:

Mislim da je to istina. – I think that it is true.
Rekao je da dolazi sutra. – He said that he is coming tomorrow.
Uče da je važno poštovati pravila. – They are learning that it is important to respect rules.

You do use commas with other kinds of da-clauses (for example some purpose or concessive clauses), but for this complement-clause use (after učiti, reći, misliti, etc.), no comma is the norm in modern Croatian spelling.

Does Djeca uče mean Children learn (in general) or Children are learning (right now)?

It can mean both, depending on context:

Habitual / general truth:
- Djeca uče da je važno poštovati pravila.
- Children (in general) are taught that it is important to respect rules.
Present ongoing action:
- With context like:
  - U školi danas djeca uče da je važno poštovati pravila.
  - Today at school, the children are learning that it is important to respect rules.

Croatian present tense of an imperfective verb (učiti) can express both simple present and present continuous, which English keeps separate. Context tells you which is intended.

What is djeca grammatically? Is it just the plural of dijete?

Djeca is the plural of dijete (child), but it is irregular:

Singular: dijete – child
Plural: djeca – children

Grammatically:

dijete is neuter singular
djeca is plural and takes plural verb agreement:
- Djeca uče. – Children learn / are learning.

You do not say:

Dijete uče – incorrect (number mismatch)
You say:
Dijete uči. – The child learns / is learning.
Djeca uče. – The children learn / are learning.