Dječak se boji koze koja ima velike rogove.

The verb is bojati se, which means to be afraid.

So se is part of the verb pattern here. It does not mean himself in this sentence; you usually just learn the whole expression bojati se as one unit.

Present tense:

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

So Dječak se boji... is the normal way to say that the boy is afraid...

Here, koze is singular: it is genitive singular of koza.

This is a very common point of confusion, because koze can also be nominative plural in other contexts.

In this sentence, we know it is singular because:

• bojati se takes the genitive
• the relative clause uses koja ima..., and ima is singular, so it refers to one goat

So here:

• koza = nominative singular
• koze = genitive singular

Because bojati se takes the genitive case.

So:

• bojati se + genitive

That gives:

• bojati se koze = to be afraid of the goat

Compare:

• Vidim kozu. = I see the goat.
  Here kozu is accusative, because vidjeti takes a direct object.
• Bojim se koze. = I am afraid of the goat.
  Here koze is genitive, because bojati se requires it.

Because the relative pronoun takes its case from its role inside the relative clause.

In koja ima velike rogove, the pronoun is the subject of ima, so it must be nominative singular feminine:

• koja = nominative singular feminine

It matches koza in gender and number, but its case comes from its function inside the clause, not from koze in the main clause.

So:

• main clause: bojati se koze → koze is genitive
• relative clause: koja ima... → koja is nominative because it is the subject of ima

It describes the goat.

The clue is koja, which is feminine singular. That matches koza.

So the structure is:

• Dječak se boji koze = The boy is afraid of the goat
• koja ima velike rogove = which has big horns

Together:

• the goat which has big horns

Because rogove is the accusative plural form, used as the direct object of ima.

The noun is:

• rog = horn

Plural forms:

• rogovi = nominative plural
• rogove = accusative plural

In this sentence, the goat has horns, so horns is the object of has:

• koja ima rogove

Compare:

• Rogovi su veliki. = The horns are big.
  Here rogovi is nominative, because it is the subject.
• Koza ima rogove. = The goat has horns.
  Here rogove is accusative, because it is the object.

Because the adjective must agree with rogove in gender, number, and case.

Here:

• rogove = masculine plural accusative
• so the adjective also takes the matching form: velike

Compare:

• veliki rogovi = big horns
  nominative plural
• ima velike rogove = has big horns
  accusative plural

So the ending changes because the noun phrase is in a different case.

Because se is a clitic, and Croatian clitics usually go in the second position of the clause.

So:

• Dječak se boji...

This is the normal order:

1. first stressed element: Dječak
2. clitic: se
3. main verb: boji

English speakers often want to put se somewhere else, but in standard Croatian, its placement is fairly restricted.

Croatian has no articles, so there is no separate word for the or a/an.

That means:

• dječak can mean the boy or a boy
• koza can mean the goat or a goat

Context decides which English translation sounds best.

So the Croatian sentence itself does not explicitly mark that difference.

In this kind of relative clause, Croatian normally keeps the relative pronoun.

So you say:

• koze koja ima velike rogove

You would not normally drop koja the way English can sometimes drop that in other sentence types.

Croatian can also express the idea differently, for example with a prepositional phrase, but in this exact structure the relative pronoun is needed.

A rough pronunciation guide is:

• Dječak ≈ DYE-chak
• se ≈ seh
• boji ≈ BO-yi
• koze ≈ KO-zeh
• koja ≈ KO-ya
• ima ≈ EE-ma
• velike ≈ VEH-lee-keh
• rogove ≈ RO-go-veh

A few useful sound notes:

• j sounds like English y
• č sounds like ch in church
• ž sounds like the s in measure

So the whole sentence is roughly:

DYE-chak seh BO-yi KO-zeh KO-ya EE-ma VEH-lee-keh RO-go-veh