Advokat čita publikaciju.

Because it is the direct object of the verb čita.

The basic dictionary form is publikacija. That is the nominative singular form.
In the sentence Advokat čita publikaciju, the noun is receiving the action of reading, so it must go into the accusative singular:

• publikacija = nominative
• publikaciju = accusative

For many feminine nouns ending in -a, the accusative singular ends in -u.

Because advokat is in the nominative form, which is the usual case for the subject.

In this sentence:

• Advokat = the one doing the action
• čita = reads / is reading
• publikaciju = the thing being read

So advokat is the subject, and publikaciju is the object.

Čita is the 3rd person singular present tense form of čitati.

So:

• čitati = to read
• čita = he/she/it reads, or is reading

Because the subject advokat is singular, the verb is also singular.

It can mean both, depending on context.

Serbian present tense often covers both:

• reads
• is reading

So Advokat čita publikaciju could mean either:

• The lawyer reads a publication
• The lawyer is reading a publication

The exact meaning depends on the situation.

Because Serbian does not have articles like English.

So Advokat čita publikaciju can mean:

• The lawyer is reading a publication
• A lawyer is reading the publication
• The lawyer is reading the publication

Context tells you which one is intended.

Yes. Serbian word order is more flexible than English because case endings show who is doing what.

The most neutral order here is:

• Advokat čita publikaciju.

But you could also hear:

• Publikaciju čita advokat.
• Čita advokat publikaciju.

These alternatives may sound more marked or place emphasis on a different word, but the endings still show the roles:

• advokat = subject
• publikaciju = object

It is accusative singular.

Here is the pattern:

• publikacija = nominative singular
• publikaciju = accusative singular

This is a very common change for feminine nouns ending in -a.

• advokat is masculine
• publikacija is feminine

This matters because gender affects things like:

• adjective endings
• pronouns
• some case endings

For example, publikacija is feminine, which is one reason its accusative singular becomes publikaciju.

A rough guide:

• č sounds like ch in chocolate
• c sounds like ts in cats
• j sounds like English y

So approximately:

• čita ≈ CHEE-ta
• publikaciju ≈ poo-blee-KA-tsee-yu

Also, Serbian spelling is very regular, so words are usually pronounced much the way they are written.

Yes, but it is a bit more formal or specific than a very general word like book or text.

Publikacija usually means something like:

• publication
• printed work
• issued text/document

So the sentence sounds natural, but somewhat neutral or formal in tone.

Yes. Advokat is a normal Serbian word meaning lawyer or attorney.

So Advokat čita publikaciju is a perfectly normal grammatical sentence. It may sound a little like a textbook example because both words are clear and easy to analyze, but it is natural Serbian.

Čitati is imperfective.

That matters because imperfective verbs are used for actions that are:

• ongoing
• repeated
• habitual
• viewed as a process

So čita fits well for is reading or reads.

If you wanted a perfective idea like read through / finished reading, Serbian would use a different verb, such as pročitati in the appropriate form.