Na stolici je nečija plava jakna.

Because na can take different cases depending on the meaning:

• na + locative = location, meaning on / at
• na + accusative = movement toward a place, meaning onto / to

So:

• Na stolici je nečija plava jakna. = There is someone’s blue jacket on the chair.
• Stavljam jaknu na stolicu. = I’m putting the jacket onto the chair.

Here the jacket is already located there, so Croatian uses the locative: stolici.

Stolici is the locative singular of stolica (chair).

The noun stolica is feminine, and its singular cases include:

• nominative: stolica
• accusative: stolicu
• locative: stolici

Since it comes after na in a location meaning, it appears in the locative.

This is a very natural Croatian word order for presenting the existence or location of something.

Na stolici je nečija plava jakna.

This structure feels a lot like English On the chair, there is someone’s blue jacket.

Croatian word order is flexible, and the choice often depends on what is already known and what is new information. Here:

• Na stolici sets the scene: on the chair
• je introduces the statement
• nečija plava jakna is the new thing being mentioned

You could also say:

• Nečija plava jakna je na stolici.

That version is also correct, but it puts more focus on the jacket rather than on the location.

Nečija means someone’s or belonging to someone.

It comes from the indefinite pronoun netko / neko (someone) and functions here like a possessive adjective.

So:

• nečiji = someone’s
• nečija = feminine singular form
• nečije = neuter singular form

In this sentence, it is nečija because it describes jakna, which is a feminine singular noun.

Because both words agree with jakna.

In Croatian, adjectives and adjective-like words must agree with the noun in:

• gender
• number
• case

Here:

• jakna = feminine singular nominative
• nečija = feminine singular nominative
• plava = feminine singular nominative

So all three match.

That is why you get:

• nečija plava jakna

If the noun were masculine, for example kaput (coat), you would get:

• nečiji plavi kaput

Jakna is in the nominative singular.

It is the grammatical subject of the sentence:

• Na stolici je nečija plava jakna.

Even though English often uses there is in this kind of sentence, Croatian still has a normal subject here: nečija plava jakna.

So jakna stays in the nominative, and the words describing it match that case.

Croatian has no articles like a/an/the.

So jakna by itself does not tell you whether it is:

• a jacket
• the jacket

You understand that from context.

In this sentence, nečija plava jakna most naturally means someone’s blue jacket, and in English you might translate the whole sentence as either:

• There is someone’s blue jacket on the chair.
• Someone’s blue jacket is on the chair.

Croatian leaves article-like distinctions to context rather than marking them directly.

Yes. Croatian word order is quite flexible.

Possible versions include:

• Na stolici je nečija plava jakna.
• Nečija plava jakna je na stolici.
• Je na stolici nečija plava jakna? (in a question-like or marked style)

But the most natural neutral version depends on emphasis.

Compare:

• Na stolici je nečija plava jakna.
  Focuses first on where something is.

• Nečija plava jakna je na stolici.
  Focuses first on what is on the chair.

So the sentence order is not random; it helps show information structure.

Je is the 3rd person singular present of the verb biti (to be).

Here it means is:

• Na stolici je nečija plava jakna.
• literally: On the chair is someone’s blue jacket.

In standard Croatian, the present tense of biti is usually:

• sam = I am
• si = you are
• je = he/she/it is
• smo = we are
• ste = you are
• su = they are

Croatian does not need a special expression exactly like English there is.

Instead, it often simply uses biti (to be) plus word order and context.

So English:

• There is a jacket on the chair.

often becomes Croatian:

• Na stolici je jakna.

Literally, that is closer to:

• On the chair is a jacket.

So the idea of there is is often expressed in Croatian through normal to be sentences.

Jakna usually means jacket.

It generally refers to a shorter outer garment, while a longer coat is more often:

• kaput = coat

So:

• plava jakna = blue jacket
• plavi kaput = blue coat

That said, exact clothing words can overlap a little depending on context, but jakna is normally best understood as jacket.

You would change the noun, adjective, possessive form, and verb to plural:

• Na stolici su nečije plave jakne.

Breakdown:

• su = are
• nečije = plural form here
• plave = plural feminine nominative
• jakne = plural nominative

So the sentence becomes:

• Na stolici su nečije plave jakne. = Someone’s blue jackets are on the chair. / There are someone’s blue jackets on the chair.