Ozbiljan korisnik će vratiti primerak.

Because će is a clitic in Serbian. Clitics usually go in the second position of the sentence or clause.

Here, the first whole unit is Ozbiljan korisnik (serious user), so će comes right after that unit:

Ozbiljan korisnik će vratiti primerak.

This is very normal Serbian word order.

This is the future tense formed with:

• the auxiliary će
• plus the infinitive vratiti

So:

• će vratiti = will return

Unlike English, Serbian does not use a separate word like to before the infinitive here. The infinitive is simply vratiti.

A useful comparison:

• On će vratiti primerak. = He will return the copy.
• Vratiće primerak. = He will return the copy. / Will return the copy.

That second version is common when the verb phrase comes first.

Because adjectives in Serbian must agree with the noun in:

• gender
• number
• case

Here, korisnik is:

• masculine
• singular
• nominative (because it is the subject)

So the adjective also has to be masculine singular nominative:

• ozbiljan korisnik

Compare:

• ozbiljna žena = serious woman
• ozbiljno dete = serious child
• ozbiljni korisnici = serious users

So ozbiljan is the correct matching form here.

Primerak is in the accusative singular because it is the direct object of vratiti.

The user is returning what?
→ primerak

For many masculine inanimate nouns in Serbian, the accusative singular looks the same as the nominative singular, so:

• nominative: primerak
• accusative: primerak

That is why its form does not change here.

Compare that with a masculine animate noun:

• nominative: korisnik
• accusative: korisnika

Because Serbian has no articles.

So a noun like korisnik can mean:

• a user
• the user

and primerak can mean:

• a copy
• the copy

The exact meaning depends on context.

If Serbian speakers want to be more specific, they can use other words, for example:

• jedan korisnik = one / a certain user
• taj korisnik = that user
• ovaj primerak = this copy

This is about aspect, which is very important in Serbian.

• vratiti is perfective
• vraćati is imperfective

Here, vratiti suggests one completed action in the future:

• će vratiti primerak = will return the copy / will give the copy back

That is the natural choice if you mean a single finished act.

By contrast, vraćati would suggest something more like:

• repeated action
• habitual action
• ongoing process

So in this sentence, vratiti is the expected verb.

Yes. Serbian word order is more flexible than English word order, but changing it usually changes the focus or emphasis.

For example:

• Ozbiljan korisnik će vratiti primerak.
  Fairly neutral.

• Primerak će vratiti ozbiljan korisnik.
  Emphasis on who will return the copy.

• Ozbiljan korisnik primerak će vratiti.
  Possible, but more marked or stylistic.

So the original sentence is a natural, straightforward version.

Not always only in the emotional sense of serious. Depending on context, ozbiljan korisnik can also suggest:

• responsible
• reliable
• genuine
• proper
• committed

So while the basic meaning is serious, the broader idea may be something like a responsible / trustworthy user.

Most often, yes, especially in modern contexts like:

• software user
• service user
• account user

But depending on context, korisnik can also mean something closer to:

• customer
• beneficiary
• recipient
• person using something

So the best English translation depends on the situation. In this sentence, user is a very likely choice, but another translation might fit better in a different context.