Kupac je kupio vozilo iako je osiguranje bilo skupo.

Because the sentence has two clauses, and each clause needs its own verb.

• Kupac je kupio vozilo = The buyer bought the vehicle
• iako je osiguranje bilo skupo = although the insurance was expensive

In the first clause, je helps form the past tense with kupio.
In the second clause, je is the present-tense form of biti (to be) used in the subordinate clause.

So the two je forms are not accidental repetition; they belong to two different parts of the sentence.

Serbian commonly forms the past tense with:

• a present form of biti (to be)
• plus the past active participle

So:

• je kupio = has bought / bought

Here:

• je = auxiliary verb
• kupio = past participle of kupiti (to buy)

This is the normal way to say the past tense in Serbian. English uses one word in bought, but Serbian uses a two-part past tense here.

Kupio agrees with the subject, kupac, which is masculine singular.

Past participles in Serbian change for gender and number:

• kupio = masculine singular
• kupila = feminine singular
• kupilo = neuter singular
• kupili = masculine plural / mixed plural
• kupile = feminine plural

So because kupac is masculine singular, the correct form is kupio.

Here vozilo is in the accusative case because it is the direct object of kupio.

The dictionary form of vozilo is nominative singular, and for many neuter nouns, the nominative and accusative singular are identical.

So:

• nominative: vozilo
• accusative: vozilo

That is why the form does not change visibly, even though its function in the sentence changes.

Iako means although / even though.

It introduces a concessive clause: a clause that expresses contrast with the main statement.

So:

• Kupac je kupio vozilo = the main fact
• iako je osiguranje bilo skupo = a contrasting fact that did not prevent the action

A natural English equivalent is:

• The buyer bought the vehicle even though the insurance was expensive.

Because osiguranje is a neuter singular noun, and the past form must agree with it.

The word bilo is the neuter singular past participle of biti (to be).

Compare:

• osiguranje je bilo skupo = the insurance was expensive
• kuća je bila skupa = the house was expensive
• auto je bio skup = the car was expensive

So bilo is used because osiguranje is neuter singular.

Because adjectives in Serbian must agree with the noun they describe in gender, number, and case.

Here the adjective describes osiguranje, which is neuter singular, so the adjective must also be neuter singular:

• masculine: skup
• feminine: skupa
• neuter: skupo

That is why the sentence has:

• osiguranje je bilo skupo

Yes. Osiguranje is neuter, and one clue is its ending: many Serbian neuter nouns end in -o or -e.

Examples:

• vozilo = neuter
• osiguranje = neuter
• more = neuter

You can also see that it behaves like a neuter noun because it takes neuter agreement:

• osiguranje je bilo skupo

Both bilo and skupo show neuter singular agreement.

Yes, Serbian word order is more flexible than English word order.

The most neutral order here is:

• Kupac je kupio vozilo iako je osiguranje bilo skupo.

But other orders are possible for emphasis or style, for example:

• Iako je osiguranje bilo skupo, kupac je kupio vozilo.

This version emphasizes the contrast first.
Even when word order changes, the cases and verb forms still show who did what.

Yes. Kupac means buyer/customer, and it is related to the verb kupiti (to buy).

This is a common pattern in Serbian: a noun can be formed from a verb to name the person who performs the action.

So:

• kupiti = to buy
• kupac = buyer

Grammatically, kupac is a masculine singular noun here and functions as the subject of the sentence.

Sometimes yes, but the meaning and style are slightly different.

• vozilo = vehicle; more general and a bit more formal
• auto = car; more everyday and more specific

So if the intended meaning is specifically car, then auto may work.
If the meaning is broader, or if the style is more formal, vozilo is the better choice.

Yes. Kupiti is a perfective verb, so it presents the action as completed.

That fits the sentence well: the buyer did buy the vehicle.

In Serbian, aspect matters:

• kupovati = imperfective, buying / used to buy / was buying
• kupiti = perfective, buy successfully / complete the purchase

So je kupio tells you the purchase was completed, not just ongoing or habitual.

In standard Serbian prose, you normally do not omit it here.

You need:

• Kupac je kupio vozilo
• osiguranje je bilo skupo

In some informal speech, auxiliaries may be reduced or dropped in special contexts, but that is not the standard form a learner should copy. For normal correct Serbian, keep both instances of je.