Korisnik će platiti zakasninu.

It is the future tense (more specifically, Future I).

In this sentence, it is formed with:

• će = the 3rd person singular future auxiliary
• platiti = the infinitive to pay

So korisnik će platiti literally works like the user will pay.

A quick pattern:

• ja ću platiti = I will pay
• ti ćeš platiti = you will pay
• on/ona/ono će platiti = he/she/it will pay

Because će is a clitic. In Serbian, clitics usually stand in the second position of the sentence or clause.

So in:

Korisnik će platiti zakasninu.

the first element is Korisnik, and the clitic će comes right after it.

If you change what comes first, će usually follows that new first element:

• Sutra će korisnik platiti zakasninu. = Tomorrow the user will pay the late fee.
• Taj korisnik će platiti zakasninu. = That user will pay the late fee.

So this is not random word order; it follows a common Serbian clitic rule.

Because zakasninu is in the accusative case, which is used here for the direct object of the verb platiti.

The dictionary form is:

• zakasnina = late fee

But after platiti (to pay), you need the thing being paid in the accusative:

• platiti zakasninu = to pay the late fee

This is a regular change for many feminine nouns ending in -a:

• nominative: zakasnina
• accusative: zakasninu

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

• platiti = perfective
  It means to pay as a completed act, once, successfully.
• plaćati = imperfective
  It means to be paying, to pay repeatedly, or to pay in general/habitually.

So:

• Korisnik će platiti zakasninu. = The user will pay the late fee.
  (one completed future action)

Compare:

• Korisnik će plaćati zakasninu.
  This would suggest repeated payment, habitual payment, or a more ongoing idea, which is probably not what is meant here.

Because Serbian does not have articles like English a/an/the.

So korisnik can mean:

• a user
• the user

and the exact meaning depends on context.

The same goes for zakasninu:

• a late fee
• the late fee

English requires an article, but Serbian usually does not.

Yes, Serbian word order is fairly flexible, because case endings show who is doing what.

The neutral order here is:

• Korisnik će platiti zakasninu.

But other orders are possible for emphasis:

• Zakasninu će platiti korisnik.
  = It is the user who will pay the late fee.

• Korisnik će zakasninu platiti.
  = Also possible, but more marked or stylistically specific.

What usually stays important is the clitic position: će still tends to come after the first element.

So word order in Serbian often affects focus and emphasis, not just grammar.

Yes. That is a very common alternative in Serbian.

So both are possible:

• Korisnik će platiti zakasninu.
• Korisnik će da plati zakasninu.

The version with the infinitive (će platiti) is very standard and common, especially in writing.

The će da + present tense pattern is also very common, especially in speech:

• će da plati

Both mean essentially the same thing here.

Grammatically, korisnik is a masculine noun. It can refer to:

• a male user specifically, or
• a user in a general/unspecified sense

If you specifically mean a female user, Serbian can use:

• korisnica = female user

So:

• Korisnik će platiti zakasninu. = The user will pay the late fee. / The male user will pay the late fee.
• Korisnica će platiti zakasninu. = The female user will pay the late fee.

Like many Slavic languages, Serbian often uses the masculine form generically unless there is a reason to specify female gender.