Roditelji se često brinu za svoju djecu, ali znaju da moraju biti hrabri.

Se is a reflexive pronoun, and with brinuti (se) it’s part of the standard verb phrase brinuti se = “to worry (about), to be concerned”.

• Brinuti se = to worry / be worried
• Brinuti nekoga = to worry someone (to cause worry to someone)

So in this meaning (“Parents often worry”), se is necessary:

• ✅ Roditelji se često brinu. = Parents often worry.
• ❌ Roditelji često brinu. – this sounds like “Parents often worry (someone)” and feels incomplete without an object.

The verb brinuti se usually takes the preposition za when it means “to worry about / care about” someone:

• brinuti se za nekoga / nešto = to worry about someone/something

So:

• ✅ brinuti se za svoju djecu = to worry about one’s children
  Without za, brinuti (nekoga) tends to mean “to worry someone” (cause them to worry):
• To me brine. = That worries me.

In your sentence, the meaning is clearly “worry about”, so you need za:
Roditelji se često brinu za svoju djecu.

Both exist, but they’re used a bit differently:

• Brinuti se za nekoga/nešto

  • Stronger idea of emotional concern or taking responsibility.
  • “To worry about; to care for (be responsible for).”
  • Brinem se za svoju djecu. = I worry about / take care of my children.

• Brinuti se o nečemu

  • More about practical care, dealing with something.
  • “To look after, to take care of (a task, topic, practical matter).”
  • On se brine o kući. = He takes care of the house.
  • Država se brine o obrazovanju. = The state takes care of education.

In the emotional sense “Parents often worry about their children”, Croatian prefers brinuti se za djecu.

Croatian has a reflexive possessive adjective svoj (with forms svoj, svoja, svoje, svoju…) which is used when the owner is the subject of the sentence.

• Subject = owner → usually use svoj
• Subject ≠ owner → use njegov, njen, njihov, etc.

Here, Roditelji (parents) are the subject and they are the ones whose children we’re talking about, so we use svoju:

• ✅ Roditelji se brinu za svoju djecu.
  = Parents worry about their (own) children.

If you say:

• Roditelji se brinu za njihovu djecu.
  it sounds like “Parents worry about their children” where “their” refers to some other people’s children (not the parents who are the subject).

The form is driven by case, gender, and number:

• Preposition za → requires accusative case.
• Djeca is an irregular noun:
  • nominative: djeca
  • accusative: djecu

The phrase is za (koga? što?) djecu → djecu is accusative.
Svoj must agree with djecu, so it takes accusative feminine singular: svoju.

That’s why we get:

• za svoju djecu (not za svoje djecu).
  You will commonly just memorize the phrase svoju djecu as the natural form.

The little word se is a clitic (an unstressed word) and in Croatian, clitics tend to go in the second position in the clause (after the first stressed word or phrase).

In Roditelji se često brinu:

• Roditelji = first stressed word
• se = clitic, placed right after it

That’s why:

• ✅ Roditelji se često brinu.
• ❌ Roditelji često se brinu. – sounds wrong to native speakers.

You can move other words:

• ✅ Često se roditelji brinu za svoju djecu.
  but se still stays in that early “second position” slot.

No; again, because of the clitic rule.

In Često roditelji se brinu:

• Često is the first stressed word.
• The clitic se wants to come right after that, but it doesn’t; it appears later, after roditelji, which is not allowed.

Correct options:

• ✅ Roditelji se često brinu za svoju djecu.
• ✅ Često se roditelji brinu za svoju djecu.
• ✅ Roditelji se brinu za svoju djecu često. (grammatically OK, but sounds a bit marked; usual place for često is before the verb).

Ali means “but” and introduces a contrast, just like English “but”:

• …brinu za svoju djecu, ali znaju…
  = “…worry about their children, but they know…”

In standard Croatian punctuation, a comma before ali is usual and natural when it connects two full clauses:

• ✅ Roditelji se često brinu za svoju djecu, ali znaju da moraju biti hrabri.

• Znaju = “they know” (3rd person plural of znati).
• da here is a conjunction meaning “that”, introducing a subordinate clause:
  
  • znaju da… = “they know that…”

Inside that clause:

• moraju = “they must / they have to” (3rd person plural of morati)
• biti = infinitive “to be”

So literally:
Znaju da moraju biti hrabri. = “They know that they must be brave.”

The pattern [finite verb] + morati + infinitive is how you say “have to do X”:

• Moram učiti. = I have to study.
• Moraš ići. = You have to go.

Hrabri is the plural masculine nominative form of the adjective hrabar (= brave).

Adjectives must agree with the noun they describe:

• roditelji (parents) → masculine plural, nominative

• So we use hrabri (masculine plural nominative):

• ✅ Roditelji su hrabri. = Parents are brave.
• ✅ … znaju da moraju biti hrabri. = … they know they must be brave.

Some contrasts:

• hrabar čovjek = a brave man (masc. sg)
• hrabra žena = a brave woman (fem. sg)
• hrabri roditelji = brave parents (masc. pl)

Present tense of brinuti se (“to worry”) is:

• ja se brinem – I worry
• ti se brineš – you (sg) worry
• on/ona/ono se brine – he/she/it worries
• mi se brinemo – we worry
• vi se brinete – you (pl/formal) worry
• oni/one/ona se brinu – they worry

Your sentence uses the 3rd person plural:

• Roditelji se često brinu… = Parents often worry…