Košulja koja je u ormaru je skupa, a jakna je na polici.

Koja means which or that here, and it refers back to košulja.

So:

• košulja = a feminine singular noun
• koja = the feminine singular form of the relative pronoun koji

In other words, koja je u ormaru means which is in the closet / that is in the closet.

A very useful pattern is:

• muški rod: koji
• ženski rod: koja
• srednji rod: koje

Because košulja is feminine, Serbian uses koja.

Because there are really two separate clauses, and each one needs its own is:

1. koja je u ormaru = which is in the closet
2. košulja ... je skupa = the shirt ... is expensive

So the sentence structure is:

• Košulja
• koja je u ormaru
• je skupa

This is completely normal in Serbian. English does the same thing:

• The shirt that is in the closet is expensive

You also have one more je later:

• jakna je na polici

So the sentence contains multiple clauses, and each clause has its own verb.

Because adjectives in Serbian agree with the noun they describe.

Here, skupa describes košulja, which is feminine singular, so the adjective must also be feminine singular.

Compare:

• skup = masculine singular
• skupa = feminine singular
• skupo = neuter singular

Examples:

• kaput je skup = the coat is expensive
• košulja je skupa = the shirt is expensive
• vino je skupo = the wine is expensive

So skupa is used because košulja is feminine.

Because this sentence expresses location, not movement.

With u:

• u + locative = in, inside, at a place
• u + accusative = into, movement toward the inside of something

Here the shirt is already located in the closet, so Serbian uses the locative:

• u ormaru = in the closet

Compare:

• Košulja je u ormaru. = The shirt is in the closet.
• Stavljam košulju u ormar. = I am putting the shirt into the closet.

So ormaru is the locative form of ormar.

For the same reason: na polici shows location, so Serbian uses the locative case.

With na:

• na + locative = on, at, on top of
• na + accusative = onto, movement onto something

So:

• jakna je na polici = the jacket is on the shelf

But if you were moving it there, you would use accusative:

• Stavljam jaknu na policu. = I am putting the jacket onto the shelf.

Here the noun polica changes in the locative singular to polici.

Here koja is in the nominative singular feminine.

Why? Because inside the relative clause, koja is the subject of je u ormaru.

The clause is essentially:

• koja je u ormaru = which is in the closet

The word koja is the thing that is in the closet, so it functions as the subject, and subjects are normally in the nominative.

This is an important point: a relative pronoun agrees with the noun it refers to in gender and number, but its case depends on its role inside the relative clause.

Here:

• refers to košulja → feminine singular
• functions as subject → nominative

So we get koja.

A often connects two clauses with a sense of contrast or switch of topic.

In many sentences it can be translated as:

• and
• while
• but

depending on context.

Here it links two different statements:

• the shirt is expensive
• the jacket is on the shelf

So a feels like a mild contrast or a shift: and / while / whereas.

It is not exactly the same as i.

• i = simple and
• a = and/but/while, often with contrast or comparison

So a is very natural here.

Because Serbian does not have articles like English.

That means Serbian usually does not have separate words corresponding to:

• a
• an
• the

Instead, definiteness is understood from context.

So košulja can mean:

• a shirt
• the shirt

depending on the situation.

The same is true for jakna, ormar, and polica.

Learners often want to add something where English uses the, but in Serbian you normally just use the noun itself.

Because Serbian often leaves the subject pronoun unstated when it is clear from context.

English needs:

• it is expensive
• it is on the shelf

But Serbian can simply say:

• je skupa
• je na polici

because the noun has already been stated:

• Košulja ... je skupa
• jakna je na polici

Serbian verbs and sentence structure often make extra pronouns unnecessary. This is very normal.

Yes, Serbian word order is more flexible than English, but not completely free.

This sentence is neutral and natural:

• Košulja koja je u ormaru je skupa, a jakna je na polici.

You could rearrange larger parts for emphasis, for example:

• Jakna je na polici, a košulja koja je u ormaru je skupa.

That changes the focus, not the basic meaning.

However, some parts should stay together:

• koja je u ormaru should stay closely connected to košulja, because it describes that noun.

So yes, word order can move for emphasis, but the relative clause still needs to clearly modify the noun it belongs to.

In standard Serbian, koja is the best choice here.

Because the relative clause refers to a specific noun, Serbian usually uses the forms of koji:

• koji
• koja
• koje

So:

• Košulja koja je u ormaru... is standard and correct.

You may sometimes hear što in speech or in certain regional/colloquial styles, but for a learner, the safest and most standard option here is koja.

So if you are describing the shirt that is in the closet, use koja.

Because a is joining two independent clauses:

• Košulja koja je u ormaru je skupa
• jakna je na polici

In Serbian punctuation, a comma is normally used before conjunctions like a when they connect full clauses.

So the comma here is standard and expected.

It helps show the sentence has two coordinated parts, not just one long phrase.