Kupac je upalio motor.

In Serbian, the usual past tense is made with:

• a present-tense form of biti (to be), here je
• plus the past active participle, here upalio

So je upalio means started / switched on in the past.

For a masculine singular subject:

• on je upalio = he started
• kupac je upalio = the customer started

In everyday speech, Serbian sometimes drops parts of biti in certain contexts, but in standard language je upalio is the normal form here.

Because the verb form agrees with the gender and number of the subject.

Here the subject is kupac (customer / buyer), which is a masculine singular noun, so the participle is:

• upalio = masculine singular
• upalila = feminine singular
• upalilo = neuter singular

Examples:

• Kupac je upalio motor.
• Kupkinja je upalila motor. or more naturally Kupica je upalila motor only in some contexts, though žena je upalila motor is simpler.
• Dete je upalilo motor.

The dictionary form is upaliti.

So:

• upaliti = to switch on / to start / to light, depending on context
• upalio = masculine singular past form

In this sentence, with motor, upaliti means to start or to switch on the engine.

Here upaliti means to start an engine.

Serbian upaliti has a broader meaning than English start. It can also mean:

• to switch on a device
• to light something, like a cigarette or a fire

Examples:

• upaliti motor = start the engine
• upaliti svetlo = switch on the light
• upaliti cigaretu = light a cigarette

So the exact English translation depends on the object.

Motor is the direct object, so it is in the accusative case.

But with masculine inanimate nouns, the accusative singular is usually the same as the nominative singular.

So:

• nominative: motor
• accusative: motor

That is why it looks unchanged.

Compare:

• Vidim motor. = I see the engine.
• Motor radi. = The engine is running.

Same form, different case function.

Because kupac is in the nominative case, which is the normal case for the subject of the sentence.

Also, the verb agrees with it:

• kupac is masculine singular
• upalio is masculine singular

That match helps show that kupac is the one doing the action.

It is in the accusative singular, because it is the direct object of upaliti.

The verb upaliti takes a direct object when you are saying what someone switched on or started:

• upaliti motor
• upaliti svetlo
• upaliti radio

As mentioned, motor looks the same in nominative and accusative because it is a masculine inanimate noun.

It can mean either, depending on context.

• customer is often the best general translation
• buyer works when the emphasis is on purchasing

So kupac je upalio motor could mean:

• The customer started the engine
• The buyer started the engine

Without more context, both are possible.

Yes, Serbian word order is more flexible than English because case endings and verb agreement help show who is doing what.

So these can all be possible:

• Kupac je upalio motor.
• Motor je upalio kupac.
• Kupac motor je upalio. (more marked, context-dependent)

But they do not all sound equally neutral.

The most neutral version here is:

• Kupac je upalio motor.

If you say:

• Motor je upalio kupac

you are likely emphasizing the customer as the person who started it, perhaps contrasting with someone else.

Also note that the clitic je normally takes the second position in the sentence, so its placement follows Serbian clitic rules.

Serbian has no articles like English a or the.

So kupac can mean:

• a customer
• the customer

and motor can mean:

• an engine
• the engine

Which one is intended depends on context.

That means the sentence could be understood as:

• A customer started the engine
• The customer started the engine

depending on the situation.

Upaliti is a perfective verb.

In Serbian, aspect is very important:

• perfective = a completed action, seen as a whole
• imperfective = an ongoing, repeated, or habitual action

The imperfective partner is usually paliti.

So:

• Kupac je upalio motor. = The customer started the engine.
  The action is completed.
• Kupac je palio motor. = The customer was starting / kept trying to start the engine.
  This sounds ongoing, repeated, or not viewed as a single completed event.

That is why upaliti is the natural choice for a simple completed past action.

A rough English-friendly pronunciation is:

• Kupac ≈ KOO-paৎস
• je ≈ ye
• upalio ≈ oo-PAH-lee-oh
• motor ≈ MOH-tor

A few notes:

• j in Serbian is pronounced like English y in yes
• c is pronounced like ts
• stress is not usually marked in normal writing, but a natural pronunciation would be close to KÚpac je upálio mótor, depending on region

Usually in a sentence like this, motor means engine or motor, so upalio motor is most naturally started the engine.

However, in informal speech, motor can sometimes refer to a motorcycle. Even then, upaliti motor still often means start the engine of that vehicle.

So the sentence most directly refers to starting an engine, but the wider context could suggest a motorcycle.

Yes. Serbian often drops subject pronouns, and sometimes even noun subjects are omitted if they are already understood from context.

So you could have:

• Je upalio motor. — possible only in very specific contexts, but not the normal standalone sentence
• Upalio je motor. — He started the engine
• Kupac je upalio motor. — explicitly says it was the customer

Because the verb form upalio already shows masculine singular, Serbian can often leave out he, but if you want to specify who, you include the noun kupac.