Dete se boji mraka.

Se is a very common Serbian clitic, often called a reflexive particle. With the verb bojati se, it is part of the verb itself: bojati se = to be afraid / to fear.

So in this sentence, you should learn the verb as a whole:

• bojati se = to fear, to be afraid

In the present tense:

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

So Dete se boji... literally follows that pattern: The child is afraid...

Because bojati se takes the genitive case, not the accusative.

• base form: mrak = darkness
• genitive singular: mraka

So:

• bojati se mraka = to be afraid of the dark/darkness

This is something learners usually just memorize with the verb:

• bojati se + genitive

Other examples:

• Bojim se psa. = I am afraid of the dog.
• Boji se visine. = He/She is afraid of heights.
• Bojimo se problema. = We are afraid of problems.

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

The subject is dete = child, which is singular, so the verb must also be singular.

Present tense of bojati se:

• ja se bojim
• ti se bojiš
• on/ona/ono se boji
• mi se bojimo
• vi se bojite
• oni/one/ona se boje

So:

• Dete se boji mraka. = The child is afraid of the dark.

In Serbian, dete is grammatically neuter, even though it refers to a person.

That is normal. Grammatical gender does not always match natural gender.

So you get:

• dete = child, neuter singular
• plural: deca = children

Because dete is singular, the verb is still singular:

• Dete se boji.

If you use a pronoun for dete, Serbian often uses neuter forms grammatically:

• Dete je malo. = The child is small.

Yes, dete is singular: a child.

Its plural is irregular:

• dete = child
• deca = children

This is a very important noun to memorize because the plural is not formed in a regular way.

Examples:

• Dete se boji mraka. = The child is afraid of the dark.
• Deca se boje mraka. = Children are afraid of the dark.

Notice how the verb changes too:

• dete se boji = singular
• deca se boje = plural

Yes, Serbian word order is fairly flexible, but se has special placement rules because it is a clitic.

The neutral, most natural order here is:

• Dete se boji mraka.

You may also see different word orders for emphasis, for example in context, but se usually stays near the beginning of the clause, often in the second position:

• Mraka se boji dete. = It is the dark that the child is afraid of.
  This is marked and used for emphasis, not the default order.

For learners, the safest pattern is:

• subject + se + verb + object/complement

So: Dete se boji mraka.

Because Serbian does not have articles like English a and the.

So dete can mean:

• a child
• the child

and mraka here can be understood as of darkness / of the dark, depending on context.

You understand whether it is definite or indefinite from the situation, context, or word order.

So Dete se boji mraka. naturally translates as:

• The child is afraid of the dark.

but literally Serbian does not include a separate word for the.

In this sentence, the most natural English translation is usually the dark.

• mrak literally means darkness or dark
• bojati se mraka idiomatically means to be afraid of the dark

So although the noun itself is darkness, the best everyday English rendering is:

• The child is afraid of the dark.

A simple pronunciation guide is:

• Dete ≈ DEH-teh
• se ≈ seh
• boji ≈ BOH-yee
• mraka ≈ MRAH-kah

A rough full pronunciation:

• DEH-teh seh BOH-yee MRAH-kah

A few useful notes:

• j in Serbian is pronounced like English y in yes
• r is rolled or tapped
• stress is not usually marked in normal writing, so learners often just imitate native audio at first

Yes. Plašiti se is another common verb meaning to be afraid.

So both are possible:

• Dete se boji mraka.
• Dete se plaši mraka.

Both mean roughly the same thing: The child is afraid of the dark.

However, the sentence you were given uses bojati se, so the main grammar point to notice is:

• bojati se + genitive

Learners will also often meet:

• plašiti se + genitive

So the case pattern is similar.

It is in the present tense, but in Serbian the present tense can express either:

1. a general state/habit
2. something true right now

So Dete se boji mraka can mean:

• The child is afraid of the dark as a general fact
• or The child is afraid of the dark right now, if the context makes that clear

Most often, without extra context, it sounds like a general statement about the child.

The practical answer is: yes, memorize it with the verb.

Some Serbian verbs regularly require certain cases, and bojati se is one of those verbs that takes the genitive.

So it is best to learn vocabulary like this:

• bojati se + genitive = to be afraid of
• plašiti se + genitive = to be afraid of

That will save you a lot of mistakes later.

Instead of memorizing only bojati se = to fear, memorize:

• bojati se nečega = to be afraid of something

Then this sentence makes immediate sense:

• Dete se boji mraka.