Brz povrat troškova bio je čudo.

The subject is the whole noun phrase Brz povrat troškova.

• brz = adjective modifying the noun
• povrat = the head noun
• troškova = a dependent noun telling you return of what? → of costs/expenses

So the verb bio je agrees with povrat, because povrat is the main noun in the subject.

Brz is the short masculine singular nominative form of the adjective, and it agrees with povrat.

Here that is the natural neutral choice:

• brz povrat = a quick return
• brzi povrat can exist, but it is the long/definite form and usually sounds more specific, marked, or less neutral here

So in an ordinary statement, brz povrat is what you would expect.

Troškova is the genitive plural of trošak (cost, expense).

Dictionary forms:

• singular: trošak
• plural: troškovi
• genitive plural: troškova

It is genitive because Croatian often uses a noun + genitive structure where English would use of:

• povrat troškova = return/reimbursement of costs
• povrat novca = return of money
• plaćanje računa = payment of bills

So troškova answers the question povrat čega? = return of what?

Because it agrees with povrat, which is a masculine singular noun.

Bio is the masculine singular past participle of biti (to be).

Compare:

• povrat → masculine singular → bio
• isplata → feminine singular → bila
• čudo → neuter singular → bilo
• troškovi → masculine plural → bili

Even though troškova appears in the sentence, the verb does not agree with that word. It agrees with the head noun povrat.

Croatian word order is more flexible than English word order.

Both of these are possible:

• Brz povrat troškova bio je čudo.
• Brz povrat troškova je bio čudo.

The version with bio je often sounds a bit more literary, formal, or stylistically shaped. The version with je bio may feel more neutral in everyday speech to many speakers.

So this is mostly a matter of word order and style, not a difference in basic meaning.

Čudo is nominative singular neuter.

After biti (to be), Croatian normally uses a predicate noun in the nominative:

• On je student.
• To je problem.
• Ovo je čudo.

So in bio je čudo, čudo is not an object. It is the noun complement of biti.

Because čudo is a noun, while čudno and čudan are adjective forms.

Here the sentence says that the quick return of costs was a miracle / a wonder:

• čudo = noun = miracle, wonder

If you used an adjective, the meaning would change:

• bio je čudan = it/he was strange
• bilo je čudno = it was strange

So čudo is correct because the sentence identifies the subject as a thing: a miracle/wonder.

Yes. It is a very normal Croatian structure, especially in formal, business, legal, and administrative language.

Croatian often uses:

• a main noun
• followed by another noun in the genitive

So:

• povrat troškova = reimbursement/return of costs
• naknada štete = compensation for damage
• prodaja kuće = sale of a house
• plaćanje poreza = payment of tax

English sometimes prefers a different phrasing, but in Croatian this noun + genitive pattern is extremely common.

Because Croatian has no articles.

So čudo can mean:

• a miracle
• the miracle

and brz povrat troškova can mean:

• a quick return of costs
• the quick return of costs

Context tells you which English article is best.

That is one of the biggest differences from English: Croatian usually leaves definiteness unstated unless something else in the sentence makes it clear.