Rezervisala sam sedište i povratnu kartu.

This is the normal Serbian past tense structure.

It is formed with:

• the past active participle: rezervisala
• the auxiliary verb jesam in clitic form: sam = I am / have in this tense pattern

So:

• rezervisala sam = I reserved / I have reserved

In Serbian, past tense often looks like this:

• radio sam = I worked (male speaker)
• radila sam = I worked (female speaker)

So rezervisala sam literally follows the same pattern.

Because the form agrees with the gender of the speaker.

In the Serbian past tense, the participle changes depending on who is speaking:

• rezervisao sam = I reserved (male speaker)
• rezervisala sam = I reserved (female speaker)

So this sentence is said by a woman.

If a man said the same sentence, it would be:

• Rezervisao sam sedište i povratnu kartu.

Because Serbian often omits subject pronouns when they are already clear from the verb.

In sam, we already know the person is I.
And in rezervisala, we also see that the speaker is female.

So ja is not necessary.

• Rezervisala sam sedište i povratnu kartu. = normal, natural
• Ja sam rezervisala sedište i povratnu kartu. = also correct, but more emphatic

You would use ja if you want contrast or emphasis, for example:

• Ja sam rezervisala sedište, a on nije.
  I reserved a seat, but he didn’t.

Because sam is a clitic, and Serbian clitics usually go in the second position of the sentence or clause.

So:

• Rezervisala sam sedište i povratnu kartu. = correct
• Ja sam rezervisala sedište i povratnu kartu. = correct

But you generally do not place sam freely wherever you want.

This second-position behavior is very common in Serbian with short forms like:

• sam, si, je, smo, ste, su
• me, te, ga, je, nas, vas, ih
• mi, ti, mu, joj, etc.

For a learner, the safest idea is:

• put the sentence together
• keep sam near the beginning, in its normal clitic slot

Sedište is the direct object of the verb rezervisati, so it is in the accusative case.

However, for many neuter singular inanimate nouns, the accusative looks the same as the nominative.

So:

• nominative: sedište = seat
• accusative: sedište = seat

That is why the form does not change.

Compare with a feminine noun like karta, which does change:

• nominative: karta
• accusative: kartu

So in this sentence:

• sedište stays sedište
• karta becomes kartu

Because this is also a direct object, so it must be in the accusative singular.

The basic dictionary form is:

• povratna karta = return ticket / round-trip ticket

But after rezervisati, it becomes accusative:

• povratnu kartu

Both words change because the adjective must agree with the noun:

• nominative: povratna karta
• accusative: povratnu kartu

This is standard adjective-noun agreement in Serbian:

• feminine singular accusative:
  • -a often becomes -u
  • povratna → povratnu
  • karta → kartu

Povratna karta means a return ticket or round-trip ticket.

• karta = ticket
• povratna = return, for coming back

So it is a ticket that includes the trip back, not just one way.

Useful contrast:

• jednosmerna karta = one-way ticket
• povratna karta = return / round-trip ticket

In the sentence, because it is the object, you get:

• jednosmernu kartu
• povratnu kartu

Because Serbian nouns belong to different genders and declension patterns, so they do not all change in the same way.

Here:

• sedište is neuter singular
• karta is feminine singular

As direct objects in the accusative:

• neuter inanimate singular often stays the same:
  • sedište → sedište
• feminine singular in -a usually changes to -u:
  • karta → kartu

So the difference is completely normal and expected.

Yes. Serbian word order is fairly flexible, though some versions sound more neutral than others.

The given sentence:

• Rezervisala sam sedište i povratnu kartu.

is a very natural neutral way to say it.

Other possible versions include:

• Rezervisala sam povratnu kartu i sedište.
• Ja sam rezervisala sedište i povratnu kartu.
• Sedište i povratnu kartu sam rezervisala.

These can differ slightly in focus or emphasis.

For example:

• Sedište i povratnu kartu sam rezervisala.
  puts more attention on what was reserved.

But for everyday use, the original sentence is the safest and most natural choice.

Yes. In this sentence, it refers to a completed past action: the reservation has already been made.

So the meaning is:

• I reserved a seat and a return ticket
• I have reserved a seat and a return ticket

In practice, Serbian past tense often covers both English ideas, and context tells you whether English would use simple past or present perfect.

The important thing for the learner is that this sentence presents the action as done.

Only the past participle would change.

A man would say:

• Rezervisao sam sedište i povratnu kartu.

A woman says:

• Rezervisala sam sedište i povratnu kartu.

Everything else stays the same.

This gender agreement in the past tense is one of the first things English speakers notice in Serbian, because English verbs do not normally change depending on whether the speaker is male or female.