Slučajno sam pozvao prijatelja.

Slučajno means accidentally, by accident, or unintentionally.

So Slučajno sam pozvao prijatelja means that the action was not deliberate. In English, you might translate the feeling as:

• I accidentally called a friend
• I called a friend by accident

It is an adverb, so it describes how the action happened.

Sam is the 1st person singular form of the auxiliary verb biti (to be) used to form the past tense.

In Serbian, the past tense is usually made with:

• the auxiliary biti
• plus the past active participle

So here:

• sam = I have / I did as part of the past tense structure
• pozvao = the past participle of pozvati

Together, sam pozvao means I called.

Pozvati is the dictionary form (the infinitive), meaning to call or to invite.

But in a normal sentence about a completed past action, Serbian uses the past participle, not the infinitive. For a masculine speaker, that form is:

• pozvati → pozvao

So:

• pozvati = to call / to invite
• pozvao sam = I called / I invited

The ending -o shows that the speaker is masculine singular.

In Serbian past tense, the participle agrees with the subject’s gender and number:

• ja sam pozvao = I called (said by a man)
• ja sam pozvala = I called (said by a woman)

So the sentence as written sounds like it is spoken by a male speaker.

Because prijatelja is in the accusative case, which is used for the direct object.

The person being called is the object of the verb, so Serbian changes the noun form.

With masculine animate nouns, the accusative singular usually looks like the genitive singular, not like the nominative:

• prijatelj = friend (nominative)
• prijatelja = friend (accusative/genitive singular)

So:

• prijatelj = a friend as the subject
• pozvao sam prijatelja = I called a friend where friend is the object

This is because sam is a clitic, and Serbian clitics usually go in second position in the sentence or clause.

That is a very common Serbian pattern.

So in:

• Slučajno sam pozvao prijatelja

the first element is Slučajno, and the clitic sam comes right after it.

You could also say:

• Ja sam slučajno pozvao prijatelja
• Slučajno sam prijatelja pozvao

But sam still tends to stay near the beginning, in the clitic position.

This can feel unusual to English speakers, because English does not have this kind of strict clitic placement.

Yes. Serbian word order is fairly flexible, although not completely free.

This sentence could appear in different orders depending on emphasis, for example:

• Slučajno sam pozvao prijatelja.
• Prijatelja sam slučajno pozvao.
• Ja sam slučajno pozvao prijatelja.

These all keep roughly the same basic meaning, but the emphasis changes:

• starting with Slučajno emphasizes the accident
• starting with Prijatelja emphasizes who was called
• adding ja can emphasize I

The version you were given is very natural and neutral.

This is mainly a question of aspect.

Serbian verbs often come in aspect pairs:

• zvati = imperfective
• pozvati = perfective

Very roughly:

• zvati focuses more on the process, repetition, or ongoing action
• pozvati focuses more on a completed single action

In this sentence, pozvao suggests a completed event: one accidental call.

That fits well with the idea I accidentally called a friend.

It can mean both:

• to call
• to invite

So pozvao sam prijatelja can sometimes mean:

• I called a friend
• I invited a friend

Context tells you which one is meant.

If the meaning already shown to the learner is called, then that is the intended reading here. But it is good to know that the verb itself is potentially ambiguous.

By itself, prijatelja does not clearly show definiteness the way English articles do.

Serbian has no articles like a and the. So depending on context, it could mean:

• a friend
• the friend
• sometimes even my friend, if the context makes that clear

If there is no extra context, English learners often translate it as a friend, but Serbian itself leaves that open.

No, that is completely grammatical.

Serbian often omits subject pronouns because the verb form already shows the person:

• sam pozvao already tells us it is I

So ja is usually unnecessary unless you want emphasis, contrast, or clarity.

Compare:

• Slučajno sam pozvao prijatelja. = neutral
• Ja sam slučajno pozvao prijatelja. = I accidentally called a friend (maybe not someone else)

A simple pronunciation guide is:

Slučajno sam pozvao prijatelja
SLOO-chai-noh sam POZ-vah-oh pree-YAH-teh-lya

A few useful notes:

• č sounds roughly like ch in church
• j sounds like English y
• lj is a soft sound, something like ly in million, though not exactly the same
• stress in Serbian is not usually marked in normal writing, so beginners often learn pronunciation by listening

If you want, you can think of it roughly as:

Sloo-CHAI-no sam po-ZVA-o pri-JA-te-lja

That is only an approximation, but it is enough to get started.