Aviokompanija nije isplatila ni naknadu štete ni povraćaj novca.

Because Serbian forms the past tense with:

• a form of biti (je, su, etc.)
• plus the l-participle (isplatio / isplatila / isplatilo)

So the positive sentence would be:

• Aviokompanija je isplatila... = The airline paid out...

When you negate je, it becomes nije:

• Aviokompanija nije isplatila... = The airline did not pay out...

So nije isplatila is the normal past-tense negative form.

Because it agrees with aviokompanija, which is a feminine singular noun in Serbian.

The past participle changes according to gender and number:

• masculine: isplatio
• feminine: isplatila
• neuter: isplatilo
• plural: isplatili / isplatile / isplatila depending on gender pattern

Since aviokompanija is feminine, Serbian uses isplatila.

Not quite.

• platiti = to pay
• isplatiti = to pay out, to disburse, to settle by payment

In this sentence, isplatila suggests that the airline did not actually issue the money that was owed. So isplatiti is especially natural for compensation, reimbursements, claims, and similar payments.

Ni ... ni ... means neither ... nor ....

So:

• ni naknadu štete ni povraćaj novca = neither compensation for damages nor a refund

In Serbian, ni is commonly used with a negative verb:

• nije ... ni ... ni ...

That combination is completely normal. It often feels like:

• did not pay either X or Y
• paid neither X nor Y

Yes, but the meaning or emphasis would change.

For example:

• Aviokompanija nije isplatila naknadu štete i povraćaj novca.

This would sound more like The airline did not pay compensation and a refund, but it is less elegant and does not emphasize the neither/nor contrast as clearly.

Using ni ... ni ... is the most natural way to say that both were not paid.

They are in the accusative case because they are the direct objects of isplatila.

The verb isplatiti answers the question what did it pay out?

• naknadu = compensation
• povraćaj = refund

So:

• isplatiti naknadu
• isplatiti povraćaj

For naknada, the accusative singular is naknadu. For povraćaj, the accusative singular is the same as the nominative: povraćaj.

Because štete is in the genitive singular.

In Serbian, nouns often combine like this:

• naknada štete = compensation for damage
• literally something like compensation of damage

The second noun often goes into the genitive to show what kind of compensation it is.

So:

• naknada = compensation
• štete = of damage / for damage

This is a very common Serbian structure.

For the same reason: Serbian often uses the genitive after a noun to show what is being returned, refunded, described, etc.

• povraćaj novca = refund of money
• more naturally in English: refund

Here:

• povraćaj = return / refund
• novca = of money

So novca is genitive singular of novac.

Yes. In many contexts, povraćaj novca is exactly refund.

Literally it is return of money, but in normal usage it often means a refund to the customer.

So in this sentence, the contrast is roughly:

• naknada štete = compensation for damage/loss
• povraćaj novca = refund of the money paid

Because Serbian has no articles.

English says:

• the airline
• a refund
• the compensation

Serbian usually expresses those ideas through context, not articles.

So:

• Aviokompanija can mean the airline or just an airline, depending on context.
• povraćaj novca can be a refund or the refund, again depending on context.

Not completely. Serbian word order is more flexible than English, but this sentence uses a very natural neutral order:

• Aviokompanija nije isplatila ni naknadu štete ni povraćaj novca.

This is straightforward and standard.

You could move things around for emphasis, but the original is probably the best default order for a learner.

Isplatiti is perfective.

That matters because perfective verbs in Serbian usually refer to a completed whole action. In a negative sentence like this, it means the completed action did not happen:

• nije isplatila = did not pay out

The imperfective partner is isplaćivati / isplaćati in some contexts, which would be used for repeated, habitual, or ongoing paying out.

So here the sentence is about the fact that the airline failed to make the payment, not about an ongoing process.

Yes. Serbian past tense often covers both ideas that English splits into:

• did not pay out
• has not paid out

Which one sounds best in English depends on context.

Without more context, either can be a reasonable translation.

Because Serbian does not need a subject pronoun when the subject is already stated.

The subject here is explicitly given:

• Aviokompanija

So there is no need to add anything like English it.

In Serbian, once the subject is clear, pronouns are often omitted even more freely than in English.

Š is pronounced like English sh in shoe.

So št is basically sht.

• štete sounds roughly like SHTEH-teh

Also:

• ć in povraćaj is a soft sound, somewhat like a very soft ch/t sound
• č and ć are different in Serbian, though many learners need time to hear and produce the contrast clearly

Yes, aviokompanija is normally written as one word.

It means airline or aviation company.

You may also see other related expressions in Serbian, but aviokompanija as one word is standard and common.