Ova torba je puna papira, ali ona mala torba je prazna.

Because they all agree with torba, which is a feminine singular noun in the nominative case.

In Serbian, words like demonstratives and adjectives usually match the noun they describe in:

• gender
• number
• case

So with torba:

• ova torba = this bag
• mala torba = small bag
• torba je puna = the bag is full
• torba je prazna = the bag is empty

If the noun were masculine or neuter, the forms would change.

They are both demonstratives, but they point to different things:

• ova = this (near the speaker)
• ona = that (farther away)

So:

• ova torba = this bag
• ona mala torba = that small bag

A useful extra note: Serbian often has a three-way distinction:

• ovaj / ova / ovo = near me
• taj / ta / to = near you or already mentioned
• onaj / ona / ono = over there, farther away

Here ona is the feminine form used with torba.

No, not in this sentence.

The word ona can mean:

• she as a pronoun
• that as a demonstrative, when it comes before a noun

Here it is followed by mala torba, so it is clearly the demonstrative:

• ona mala torba = that small bag

If it were a pronoun, it would stand on its own more like Ona je prazna = It/She is empty. But with torba right after it, the meaning is that.

Because pun / puna / puno normally takes the genitive case in Serbian.

So:

• pun nečega = full of something

That is why you get:

• puna papira = full of paper / papers

The noun papir changes into the genitive form papira after puna.

This is a very common pattern:

• čaša puna vode = a glass full of water
• soba puna ljudi = a room full of people

It can be understood either way depending on context.

Papira can represent:

• genitive singular of papir in a mass sense: full of paper
• or a plural idea in context: full of papers

In everyday use, Serbian often treats papir as a material or mass noun, so puna papira can naturally mean full of paper or full of papers/documents, depending on the situation.

Because each clause has its own verb:

• Ova torba je puna papira
• ali ona mala torba je prazna

The word je is the 3rd person singular present of biti = to be.

English also repeats the verb here:

• This bag is full of paper, but that small bag is empty.

So Serbian does the same.

That order is very normal in Serbian:

• demonstrative + adjective + noun
• ona mala torba = that small bag

This is the most straightforward way to build the phrase.

Serbian word order is more flexible than English, but not every order sounds equally neutral. In a basic descriptive sentence, ona mala torba is the natural choice.

Because they are functioning as predicate adjectives.

The structure is:

• torba je puna
• torba je prazna

This is like English:

• the bag is full
• the bag is empty

So puna and prazna are not directly before the noun here; they describe the subject after the verb to be.

Compare:

• puna torba = a full bag
• torba je puna = the bag is full

Yes, very often it could.

You could say:

• Ova torba je puna papira, ali ona mala je prazna.

That would still sound natural, because torba is understood from context.

Repeating torba is also correct. It can make the sentence feel a bit clearer or more explicit, especially for learners or in careful speech.

Ali means but.

It connects two contrasting ideas:

• this bag is full
• that small bag is empty

So it works just like English but:

• Ova torba je puna papira, ali ona mala torba je prazna.

Only partly. The basic pieces match well, but not everything works exactly word-for-word.

Roughly:

• Ova = this
• torba = bag
• je = is
• puna papira = full of paper/papers
• ali = but
• ona mala torba = that small bag
• je prazna = is empty

The main thing that does not match English literally is puna papira, because Serbian uses the genitive case after puna, while English uses of:

• full of paper
• Serbian: puna papira