Rezervisaću sedište čim kupim kartu.

Čim means as soon as. It shows that the second action happens immediately after the first one.

So:

• čim kupim kartu = as soon as I buy the ticket
• kad kupim kartu = when I buy the ticket

Both can refer to the future, but čim is more immediate and specific about sequence.

In Serbian, after time words/conjunctions like čim, kad, posle što, and similar expressions, a present-tense form is often used even when the meaning is future.

So:

• Rezervisaću sedište čim kupim kartu.

literally contains kupim as a present form, but the whole meaning is future because the main clause is future and the context is clear.

This is actually similar to English:

• I’ll reserve the seat as soon as I buy the ticket.

English also uses present buy, not will buy, after as soon as.

Because kupim comes from kupiti, which is a perfective verb. It refers to a single completed action: buying the ticket.

• kupim = I buy / I complete the purchase
• kupujem = I am buying / I buy habitually / I’m in the process of buying

After čim, Serbian often prefers a perfective present when one action must be completed before the next one happens.

So here the meaning is:

• first I finish buying the ticket
• then I reserve the seat

That is why kupim is the natural choice.

Rezervisaću is the 1st person singular future form of rezervisati.

It is formed from:

• rezervisati = to reserve
• ću = I will

When the future auxiliary comes after the infinitive in Serbian, the infinitive ending -ti is dropped and the form joins together:

• rezervisati + ću → rezervisaću

So rezervisaću means I will reserve.

Yes, you can also say Ja ću rezervisati sedište čim kupim kartu.

Both are correct:

• Rezervisaću sedište čim kupim kartu.
• Ja ću rezervisati sedište čim kupim kartu.

The difference is mainly in structure and emphasis:

• rezervisaću = the future auxiliary is attached to the verb
• ću rezervisati = the auxiliary comes before the infinitive

Also, ja is usually omitted unless you want emphasis or contrast, because the verb form already shows the person.

So the original sentence is a very normal, natural way to say it.

Serbian often leaves out subject pronouns when they are already clear from the verb form.

Here, -ću tells you that the subject is I.

So:

• Rezervisaću already means I will reserve

You only add ja if you want emphasis, contrast, or clarity:

• Ja ću rezervisati sedište, a ti kupi kartu.

Without special emphasis, leaving ja out is completely normal.

Because kartu is the accusative singular form of karta.

In this sentence, kartu is the direct object of kupim:

• kupim šta? → kartu

So:

• nominative: karta
• accusative: kartu

This is a very common feminine noun pattern in Serbian.

Because sedište is a neuter noun, and in the singular its nominative and accusative forms are the same.

Here, sedište is also a direct object:

• rezervisaću šta? → sedište

But for this noun, the form does not change:

• nominative: sedište
• accusative: sedište

So the case is still accusative, even though the word looks the same.

The dictionary forms are:

• rezervisati = to reserve
• kupiti = to buy
• čim is not a verb; it is a conjunction meaning as soon as

The actual forms in the sentence are:

• rezervisaću = I will reserve
• kupim = I buy in form, but here used with future meaning after čim

Yes. Serbian word order is fairly flexible, and changing it often changes emphasis rather than basic meaning.

For example:

• Rezervisaću sedište čim kupim kartu.
  Neutral, natural order.

• Čim kupim kartu, rezervisaću sedište.
  Puts more focus on the time sequence.

• Sedište ću rezervisati čim kupim kartu.
  Puts more emphasis on sedište.

All of these can be correct, but the original sentence sounds very natural and neutral.

Here are the key sounds:

• č in čim = like ch in church, but usually a bit stronger
• ć in rezervisaću = a softer sound, somewhat between English t and ch; English does not have an exact equivalent
• š in sedište = sh as in ship

A rough pronunciation guide for the whole sentence is:

re-ze-rvi-SA-ću SE-di-shte chim KU-pim KAR-tu

That is only approximate, but it can help you get started.