Upalila sam motor.

Upalila is the past participle form of the verb, and it agrees with the speaker's gender.

• upalila sam = I did it, and the speaker is female
• A male speaker would say upalio sam motor

So this sentence specifically tells you that the person speaking is a woman.

Sam is the 1st person singular present tense of biti (to be), and it is used to form the past tense in Serbian.

So the past tense is built like this:

• upalila = past participle
• sam = auxiliary verb am

Together:

• Upalila sam = I started / turned on

This is similar in function to compound past forms in many European languages, though in English we usually just say I started rather than I have started in this kind of sentence.

Because sam is a clitic in Serbian. Clitics usually go in the second position of the sentence or clause.

That is why you commonly see:

• Upalila sam motor.
• Ja sam upalila motor.

but not normally:

• Sam upalila motor.

So even though sam translates as part of the verb phrase, its position follows Serbian clitic placement rules.

Yes. That is completely correct.

Serbian often drops the subject pronoun when it is already clear from the verb form.

So:

• Upalila sam motor. = neutral, natural
• Ja sam upalila motor. = I started the engine, with extra emphasis on I

You add ja when you want contrast, emphasis, or clarity.

Motor is in the accusative case, because it is the direct object of the verb.

The verb upaliti takes an object: you start or turn on something.

Here is the useful detail:

• motor is a masculine inanimate noun
• For many masculine inanimate nouns, nominative and accusative singular look the same

So:

• nominative: motor
• accusative: motor

That is why the form does not change here.

It can mean all of those, depending on the object.

Common meanings of upaliti include:

• start an engine: upaliti motor
• turn on a device/light: upaliti televizor, upaliti svetlo
• light a cigarette/fire: upaliti cigaretu, upaliti vatru

So in this sentence, with motor, the natural meaning is started the engine.

Upaliti is perfective.

That means it usually refers to a completed action: the engine was successfully started.

Its usual imperfective partner is paliti.

Compare:

• Upalila sam motor. = I started the engine.
  completed event
• Palila sam motor. = I was starting / used to start the engine.
  process, repetition, or background action

This aspect difference is very important in Serbian.

Because paliti and upaliti do not mean exactly the same thing.

• upaliti = complete the action successfully, start/turn on
• paliti = be in the process of doing it, do it repeatedly, or talk about the action more generally

So if you mean a single completed event, Upalila sam motor is the normal choice.

If you said Palila sam motor, it could sound like:

• I was trying/starting the engine
• I used to start the engine
• I was in the middle of starting it

In the singular, very often yes.

Examples with the same verb:

• upalio sam = I started, male speaker
• upalila sam = I started, female speaker

For you/he/she, gender is also visible in the past participle:

• upalio si = you started, male
• upalila si = you started, female
• upalio je = he started
• upalila je = she started

So Serbian past tense gives you gender information that English does not normally show.

Not usually in this exact form.

In Serbian, motor often means engine rather than motorcycle.

So:

• Upalila sam motor. usually means I started the engine

If you want to say motorcycle, the more common word is:

• motorcikl
• colloquially sometimes motor, depending on context

But without context, learners should understand motor here as engine.

Because Serbian has no articles like English a/an/the.

So motor can mean:

• an engine
• the engine

The exact meaning depends on context.

That is normal in Serbian, and learners have to get used to understanding definiteness from the situation rather than from an article.

Yes, very natural.

A Serbian speaker would absolutely say:

• Upalila sam motor.

if she wants to say she started the engine.

It is short, normal, and idiomatic. The only thing to watch is the speaker's gender:

• female speaker: Upalila sam motor.
• male speaker: Upalio sam motor.