Learn Hardcore Serbian: Advokat čita publikaciju. - The lawyer reads the publication.

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.

How do I know that advokat is the subject?

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.

What form of the verb is čita?

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.

Does čita mean or ?