Moja sestra se boji mraka.

Because the verb is bojati se, not just bojati in this meaning.

In Croatian, many verbs are learned together with se because they function as a unit. Bojati se means to be afraid / to fear. So:

• bojim se = I am afraid
• bojiš se = you are afraid
• boji se = he/she is afraid

Here, se is a short clitic form, and it normally appears near the beginning of the sentence, often in second position.

So Moja sestra se boji mraka uses the full verb pattern correctly.

Boji is the 3rd person singular present tense form of bojati se.

The subject is moja sestra = my sister, which is she, so the verb must be in the he/she/it form:

• ja se bojim = I am afraid
• ti se bojiš = you are afraid
• on/ona se boji = he/she is afraid

So boji matches sestra perfectly.

Because bojati se takes the genitive case.

The basic noun is:

• mrak = darkness / dark

But after bojati se, Croatian uses the genitive:

• bojati se mraka = to be afraid of the dark

So:

• mrak = nominative
• mraka = genitive singular

This is one of the most important things to memorize with the verb:
bojati se + genitive

Because Croatian often expresses that kind of relationship through case endings instead of a separate word.

In English, you say:

• afraid of the dark

In Croatian, the genitive ending on mraka does the job that of does in English.

So the idea of of the dark is built into mraka itself.

Both are in the nominative singular, because moja sestra is the subject of the sentence.

• sestra = sister
• moja = my

Since sestra is a feminine singular noun, the possessive word must agree with it:

• masculine singular: moj
• feminine singular: moja
• neuter singular: moje

So:

• moj brat = my brother
• moja sestra = my sister
• moje dijete = my child

It can cover the ideas of darkness or the dark, depending on context.

In this sentence, mraka is best understood as:

• of the dark
• or of darkness

Croatian has no articles like the or a, so you do not see a separate word corresponding to the.

That is why mraka can naturally be translated as the dark in English.

Yes, Croatian word order is more flexible than English, but this version is the most neutral and natural.

A key point is that se is a clitic, and clitics usually come very early in the sentence, often in second position.

So:

• Moja sestra se boji mraka. = neutral, standard

You may also hear other orders in context, for emphasis, for example:

• Sestra se boji mraka.
• Mraka se boji moja sestra. = emphasizes mraka

But for learners, the given sentence is an excellent default pattern.

A simple approximate pronunciation is:

• boji ≈ BOH-yi
• mraka ≈ MRAH-kah

A few helpful details:

• j in Croatian sounds like English y in yes
• r is rolled or tapped
• vowels are usually clear and short, not heavily reduced like in English

So boji is two syllables: bo-ji, not one long English-style vowel sound.

Not exactly.

It is often called a reflexive or pronominal verb because it uses se, but it does not literally mean fear oneself.

In this sentence, se is just part of how the verb is used idiomatically. So it is better to learn:

• bojati se = to be afraid

as one vocabulary item.

That is often the safest approach with Croatian verbs that use se.

You put ne before the verb:

• Moja sestra se ne boji mraka. = My sister is not afraid of the dark.

Notice that se stays in its usual clitic position, and ne goes with the verb:

• se ne boji

That order is normal and natural.

You would need to change both the subject and the verb:

• Moje sestre se boje mraka.

Changes:

• moja sestra → moje sestre = my sisters
• boji → boje = are afraid

The object stays in the genitive:

• mraka

So the pattern is still:

subject + se + bojati se-form + genitive