Kupac će platiti poštarinu pouzećem.

Kupac is in the nominative singular. It is the subject of the sentence: the person who will do the action.

For many masculine nouns in Serbian, the nominative singular is the basic dictionary form, so kupac appears unchanged here.

Because će is a clitic: a short, unstressed word that normally goes in second position in the clause.

So in:

Kupac će platiti poštarinu pouzećem.

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

This is very normal in Serbian. Compare:

• Kupac će platiti...
• Sutra će kupac platiti...

In both cases, će tends to stay near the beginning, after the first phrase.

This is the Serbian future I tense.

It is formed with:

• a present-tense clitic form of hteti (ću, ćeš, će, ćemo, ćete, će)
• plus the infinitive of the main verb

So here:

• će = will
• platiti = infinitive to pay

Together: će platiti = will pay

With a subject before it, Serbian normally keeps them separate:

• Kupac će platiti

If the verb phrase comes first, Serbian often uses the merged spelling:

• Platiće poštarinu.

Because platiti is perfective, and plaćati is imperfective.

In this sentence, the payment is understood as one complete action, so platiti is the natural choice.

• platiti = to pay, viewed as completed
• plaćati = to be paying / to pay repeatedly / habitually

So:

• Kupac će platiti... = the buyer will pay
• Kupac će plaćati... would suggest repeated or ongoing payment, which does not fit as well here

Because poštarinu is in the accusative singular, not the nominative.

The verb platiti takes a direct object, and direct objects are usually in the accusative.

The dictionary form is:

• poštarina = nominative singular

But as the object of platiti, it becomes:

• poštarinu = accusative singular

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

• knjiga → knjigu
• cena → cenu
• poštarina → poštarinu

Pouzećem is the instrumental singular of pouzeće.

Here the instrumental expresses the means or method by which the payment happens. So the idea is:

• platiti pouzećem = pay by cash on delivery

This use of the instrumental is very common in Serbian for expressing how something is done.

The forms are:

• pouzeće = nominative singular
• pouzećem = instrumental singular

Because Serbian often uses the instrumental without a preposition to express means, method, or manner.

So Serbian can say:

• platiti karticom = pay by card
• platiti gotovinom = pay in cash
• platiti pouzećem = pay by cash on delivery

To an English speaker, this can feel unusual because English often needs by, with, or in, but Serbian often does not.

Yes. Serbian word order is fairly flexible because the cases show what each noun is doing.

The neutral order here is:

Kupac će platiti poštarinu pouzećem.

But other orders are possible, depending on emphasis:

• Poštarinu će kupac platiti pouzećem.
• Kupac će pouzećem platiti poštarinu.

These versions are still understandable, but they sound more marked and put focus on different parts of the sentence. The original version is the most straightforward and neutral.

Because Serbian has no articles.

So kupac can mean:

• the buyer
• a buyer
• sometimes just buyer in a general sense

The exact meaning depends on context, not on a separate word like the or a.

That is one of the big differences from English.