Kum je već nazvao mladoženju, jer se boji da će zaboraviti prsten u autu.

In a wedding context, kum usually means the best man.

Be careful, though: kum can also mean godfather in other contexts, so it is a culturally broader word than English best man. In this sentence, because of mladoženju (the groom) and prsten (the ring), the wedding meaning is the natural one.

Je nazvao is the Croatian perfect tense:

• je = auxiliary to be in 3rd person singular
• nazvao = past participle

So kum je nazvao means the best man called / has called.

Also, nazvati is a perfective verb, so it presents the call as a completed action. That is why it fits well with već (already).

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

So:

• Kum je već nazvao mladoženju = normal
• not usually Kum već je nazvao...

This same second-position tendency affects other short words too, like se, ću/će, and short pronouns.

Because mladoženju is the accusative singular form.

Here, mladoženju is the direct object of nazvao:

• nominative: mladoženja = the groom
• accusative: mladoženju = the groom, as the object

This happens because masculine animate nouns usually change form in the accusative.

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

So se is not optional here; it is part of the verb itself:

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

Many Croatian verbs are learned together with se, and this is one of them.

After verbs like bojati se (to be afraid), Croatian often uses a da-clause:

• boji se da... = is afraid that...

Then the future is formed with će + infinitive:

• će zaboraviti = will forget

So:

• boji se da će zaboraviti = is afraid that he will forget

This is very natural Croatian structure.

Yes, it can be ambiguous.

In da će zaboraviti prsten u autu, the subject is not stated explicitly. Croatian often leaves subjects unstated when they are understood from context.

So the sentence could mean:

• the best man is afraid he himself will forget the ring, or
• the best man is afraid the groom will forget it

Context decides.

If you want to make it clear, Croatian can name the subject:

• ...jer se boji da će mladoženja zaboraviti prsten u autu.
• ...jer se boji da će on zaboraviti prsten u autu.

Prsten is also a direct object, so it is in the accusative. But for many masculine inanimate nouns, the accusative looks the same as the nominative.

So:

• nominative: prsten
• accusative: prsten

That is normal.

Also, Croatian has no articles like a or the, so prsten can mean a ring or the ring, depending on context.

Because this sentence describes location, not movement.

With u:

• u + accusative often means into (motion)
• u + locative means in/inside (location)

Here the ring is in the car, so Croatian uses the locative:

• u autu = in the car

Compare:

• Stavio je prsten u auto. = He put the ring into the car.
• Prsten je u autu. = The ring is in the car.

Već means already.

It suggests that the action has happened by now or earlier than expected:

• Kum je već nazvao mladoženju = The best man has already called the groom.

It often adds a small sense of urgency, progress, or expectation.

Croatian is a pro-drop language, which means subject pronouns are often omitted when they are clear from the verb form or context.

So instead of saying On je već nazvao..., Croatian can simply say:

• Kum je već nazvao...

Likewise, later clauses often leave the subject unstated too:

• se boji
• će zaboraviti

If you add on, it usually gives extra emphasis or helps remove ambiguity.

Yes. Both can mean because.

So this is also possible:

• Kum je već nazvao mladoženju, zato što se boji da će zaboraviti prsten u autu.

A few notes:

• jer is shorter and very common
• zato što is also common, sometimes a bit heavier or more explicit
• the comma before the reason clause is normal in standard writing

Both are good Croatian.