Iako je vozilo polovno, motor radi dobro.

Iako means although / even though. It introduces a concession clause: a clause that presents a fact that contrasts with what follows.

So the structure is:

Iako + clause, main clause

In this sentence:

• Iako je vozilo polovno = although the vehicle is used
• motor radi dobro = the engine works well

It is very common to put the iako clause first, but it can also come later:

• Motor radi dobro, iako je vozilo polovno.

Both are natural.

This is because je is a clitic in Serbian. Clitics are short unstressed words that usually go in the second position of their clause.

In Iako je vozilo polovno:

• Iako is the first element
• je comes right after it
• then the rest follows

This second-position rule is very important in Serbian and applies to forms like:

• je, su, sam, si, smo, ste
• short object pronouns like ga, je, mu, joj, etc.

So Iako vozilo je polovno sounds wrong in standard Serbian.

Because vozilo is a neuter singular noun, and the adjective polovno must agree with it in:

• gender
• number
• case

Here:

• vozilo = neuter singular
• polovno = neuter singular adjective form

Compare:

• polovan automobil = a used car (masculine)
• polovna kola = used cars / a used car, depending on meaning (plural/neuter plural form in everyday usage with kola)
• polovno vozilo = a used vehicle (neuter)

This agreement is a basic feature of Serbian adjectives.

Both are possible, but they are not exactly the same stylistically.

• polovno vozilo is the more neutral, usual order: adjective + noun
• vozilo polovno places more focus on the adjective, almost like saying the vehicle is used

In your sentence, vozilo polovno appears after je, so it functions like the predicate part of to be:

• vozilo je polovno = the vehicle is used

That is why the adjective comes after the noun here.

Yes, very similar.

The pattern is:

• subject: vozilo
• copula: je
• predicate adjective: polovno

So vozilo je polovno literally matches the vehicle is used.

Serbian often uses adjectives this way after forms of biti = to be.

Vozilo means vehicle, so it is broader and more general.

It can refer to:

• a car
• a truck
• a van
• other kinds of vehicles

Automobil specifically means automobile / car.

So this sentence sounds a bit more general or formal with vozilo. In everyday speech, many speakers might say:

• Iako je auto polovan, motor radi dobro.

That would sound more conversational.

In Serbian, raditi can mean to work, to function, or for a machine/engine, to run.

So motor radi dobro means:

• the engine works well
• the engine runs well
• the engine is functioning properly

This is completely natural Serbian.

Here radi is the 3rd person singular present of raditi because motor is singular.

Because dobro here is an adverb, not an adjective.

It modifies the verb radi:

• radi dobro = works well

If you used dobar, that would be an adjective meaning good, and it would need to describe a noun, such as:

• dobar motor = a good engine

So:

• motor je dobar = the engine is good
• motor radi dobro = the engine works well

That is the key difference.

Because motor is the subject of the main clause, so it is in the nominative case.

In motor radi dobro:

• motor = subject
• radi = verb
• dobro = adverb

Likewise, vozilo is also nominative in the subordinate clause because it is that clause’s subject:

• vozilo je polovno

So both nouns are in nominative because each one is the subject of its own clause.

No special case is required just because of iako.

What iako does is introduce a subordinate clause, but the grammar inside that clause is otherwise normal. In your sentence:

• vozilo stays nominative because it is the subject
• je is the present tense of biti
• polovno agrees with vozilo

So unlike some prepositions, iako does not govern a case.

Yes, in standard writing, the comma is expected here.

The sentence starts with a subordinate clause:

• Iako je vozilo polovno, ...

Then the main clause follows:

• ... motor radi dobro.

Serbian normally separates this type of subordinate clause with a comma, especially when it comes first.

So the punctuation in your sentence is standard and correct.

Yes, but not every change sounds equally natural.

Natural possibilities include:

• Iako je vozilo polovno, motor radi dobro.
• Motor radi dobro, iako je vozilo polovno.
• Iako je polovno vozilo, motor radi dobro.
  This is possible, but less neutral than the original.

What you generally should not do is move the clitic je freely, because clitic placement is restricted.

For example, Iako vozilo je polovno is not standard.

In standard Serbian, no. You need je in this sentence.

• vozilo je polovno = correct
• vozilo polovno = incomplete in standard grammar if you mean the vehicle is used

Serbian can sometimes omit forms of to be in very informal or special styles, but that is not what is happening here. A learner should treat je as required.

Usually yes in practical meaning, but the tone is a little flexible.

Polovan / polovno commonly means:

• used
• second-hand
• not new

For vehicles, it is a very common word. So polovno vozilo is exactly the kind of phrase you would expect in ads, paperwork, or general description.

It does not sound strange at all.

The sentence uses the present tense:

• je = is
• radi = works / runs

This is normal because the sentence describes a current state and a current fact:

• the vehicle is used
• the engine works well

Serbian often uses the present tense just as English does for general present facts like this.