Prema opisu, stvar je nekorišćena.

Here prema means according to or judging by. In this use, it takes the dative case.

So:

• opis = description
• opisu = to/according to the description in the dative singular

In other contexts, prema can also mean toward, but in this sentence it means according to.

Because prema requires the dative.

The base form is:

• opis = nominative singular

After prema, it becomes:

• opisu = dative singular

So prema opisu literally works like according to the description.

No.

• prema opisu = according to the description
• u opisu = in the description

That is an important distinction. The sentence is not saying where the information appears; it is saying what conclusion is being drawn from the description.

Stvar is a very general word meaning thing, item, or object.

In this sentence, it refers to the item being discussed. It is a bit generic. Depending on context, Serbian might also use:

• predmet = object / item more formal
• artikl = item / product
• roba = goods / merchandise

Also note that stvar is a feminine noun, which matters for agreement later in the sentence.

Because nekorišćena agrees with stvar, which is:

• feminine
• singular
• nominative

So the adjective/participle must also be feminine singular:

• masculine: nekorišćen
• feminine: nekorišćena
• neuter: nekorišćeno

Since stvar is feminine, nekorišćena is the correct form.

It is originally a past passive participle from koristiti = to use, but in sentences like this it often functions very much like an adjective.

Breakdown:

• koristiti = to use
• korišćen = used
• nekorišćen / nekorišćena = unused

So stvar je nekorišćena means the item is in an unused state.

A very similar alternative is:

• stvar nije korišćena = the item was not used / has not been used

Both are possible, but the structure is slightly different.

Je is the 3rd person singular present form of biti = to be.

The sentence has an adjectival predicate:

• stvar = subject
• je nekorišćena = is unused

So je is simply the Serbian equivalent of is here.

Because je is a clitic, and Serbian clitics usually appear near the beginning of their clause, often in the second position.

The main clause is:

• stvar je nekorišćena

That order is natural and standard.

The opening phrase Prema opisu is an introductory phrase set off by a comma, so the clitic placement is determined by the main clause that follows.

It is very natural here because Prema opisu is an introductory phrase.

So:

• Prema opisu, stvar je nekorišćena.

looks polished and clear.

In informal writing, people might omit the comma, but using it is a good choice.

Yes, it is understandable and grammatical. It sounds somewhat neutral or slightly formal.

Depending on context, especially in ads or listings, speakers might also say things like:

• Prema opisu, predmet nije korišćen.
• Po opisu, stvar nije korišćena.

Your sentence is fine, but stvar is fairly general, so another noun may sometimes sound more specific.

Roughly like:

• ne-ko-rish-che-na

A few sounds to notice:

• š sounds like sh
• ć is a soft sound, somewhat like a very soft ty/ch
• the cluster šć can be tricky for English speakers

So korišćena is not pronounced like plain English korishena; it has that extra soft ć sound in the middle.

Yes. Stvar is a very flexible word in Serbian. It can mean:

• thing
• matter
• issue
• affair

For example:

• To je druga stvar. = That is another matter.

But in your sentence, because it is described as nekorišćena, the meaning is clearly a physical item/object.