Težak akumulator ostaje izvan auta.

Because akumulator is a masculine singular noun in the nominative case, and the adjective has to agree with it.

• težak = masculine singular
• teška = feminine singular
• teško = neuter singular

So:

• težak akumulator = a heavy battery

This is standard adjective-noun agreement in Serbian.

It is in the nominative singular because it is the subject of the sentence.

The sentence structure is basically:

• Težak akumulator = subject
• ostaje = verb
• izvan auta = prepositional phrase

So akumulator is the thing doing the action of remaining/staying.

ostaje is the present tense, 3rd person singular form of the verb.

• ostati = infinitive, to remain / to stay
• ostaje = remains / stays

Since the subject is singular (akumulator), the verb is also singular:

• akumulator ostaje = the battery remains

So Serbian uses a conjugated verb here, just like English uses remains rather than to remain.

Because izvan requires the genitive case, and the genitive singular of auto is auta.

So:

• nominative: auto
• genitive: auta

That is why you get:

• izvan auta = outside the car

This is a very common thing in Serbian: certain prepositions require a specific case, and izvan takes the genitive.

Yes, izvan is normally used with the genitive.

Examples:

• izvan kuće = outside the house
• izvan grada = outside the city
• izvan auta = outside the car

So once you see izvan, it is a good idea to expect the next noun to be in the genitive.

Izvan means outside of / outside and is a standard, somewhat formal-neutral preposition.

You may also hear van, which often has a similar meaning:

• izvan auta
• van auta

Both can mean outside the car.

A learner should know that izvan is completely correct and standard. In everyday speech, van is also very common.

Yes, automobila is also possible if you use automobil instead of auto.

Compare:

• izvan auta = outside the car
• izvan automobila = outside the automobile/car

Both are grammatically fine.
The difference is mainly vocabulary choice:

• auto = shorter, very common in everyday speech
• automobil = more formal/full form

It can correspond to either remains or stays, depending on context.

In this sentence, ostaje suggests that the battery continues to be outside the car or is left outside the car.

So in English, depending on context, you might translate it as:

• The heavy battery remains outside the car.
• The heavy battery stays outside the car.

The Serbian verb ostati / ostajati often carries the idea of remaining after some decision, change, or contrast.

Because Serbian does not have articles like English a/an/the.

So:

• težak akumulator can mean a heavy battery or the heavy battery

Which one is meant depends on the context.

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

Yes, Serbian word order is relatively flexible, although some orders sound more neutral than others.

The given sentence:

• Težak akumulator ostaje izvan auta.

is a normal, neutral order.

Other orders are possible for emphasis, for example:

• Akumulator ostaje izvan auta.
• Izvan auta ostaje težak akumulator.
• Težak akumulator izvan auta ostaje. (possible, but less neutral and more stylistically marked)

In everyday learning, it is best to treat Težak akumulator ostaje izvan auta as the straightforward default pattern.

In this sentence, yes, it is understood as physically heavy.

The adjective težak can also mean things like:

• difficult
• hard
• serious

But with akumulator, the natural reading is physical weight:

• težak akumulator = a heavy battery

So the noun usually makes the meaning clear.

Yes, auto is neuter.

That is why its nominative singular is:

• auto

And its genitive singular is:

• auta

So even though the subject of the sentence is masculine (akumulator), the noun inside the prepositional phrase is neuter:

• izvan auta

This is a useful reminder that each noun keeps its own gender and declines accordingly.

Yes, it can.
Because ostaje often suggests that something continues to be in a certain place/state, it may imply contrast with another possibility.

So Težak akumulator ostaje izvan auta could mean something like:

• the battery is not being put into the car
• the battery is being left outside
• the battery remains outside while something else happens

That nuance often comes from context rather than from the sentence alone.