U sobi je moja knjiga.

Because u can take different cases depending on meaning:

• u + locative = location, in / inside
• u + accusative = motion toward, into

Here the sentence describes where the book already is, so Serbian uses the locative:

• u sobi = in the room
• u sobu = into the room

So U sobi je moja knjiga means the book is already there, not moving there.

Sobi is locative singular.

The noun is soba = room, and for a feminine noun like this, the locative singular form is:

• soba → sobi

So:

• u sobi = in the room

This is a very common pattern with feminine nouns ending in -a.

Because moja knjiga is the subject of the sentence, and subjects are normally in the nominative case.

• knjiga = nominative singular
• moja matches it in gender, number, and case

So:

• moja knjiga = my book (feminine singular nominative)

If it were a direct object, the form would be different:

• Vidim moju knjigu. = I see my book.

But here the book is not an object; it is the thing that is somewhere.

Because knjiga is a feminine noun.

In Serbian, possessive words like my change to agree with the noun:

• moj = masculine
• moja = feminine
• moje = neuter

Examples:

• moj pas = my dog
• moja knjiga = my book
• moje selo = my village

Since knjiga is feminine, the correct form is moja.

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

So in this sentence it means is:

• U sobi je moja knjiga. = My book is in the room.

It agrees with knjiga, which is singular.

This is because je is a clitic, and Serbian clitics usually go in the second position of the sentence or clause.

In U sobi je moja knjiga:

1. U sobi is the first unit.
2. je comes right after it.
3. Then comes moja knjiga.

So the order is not random. Serbian often places short words like je, sam, si, ga, se, etc. in this special second position.

That is why U sobi moja knjiga je sounds wrong in normal Serbian.

Yes, absolutely.

Both are correct, but the emphasis is slightly different:

• Moja knjiga je u sobi. = a more neutral My book is in the room
• U sobi je moja knjiga. = puts in the room first, often because the location is the starting point or the speaker is highlighting what is in the room

Serbian word order is more flexible than English, so changing the order often changes focus rather than basic meaning.

It is normal and natural.

It may sound slightly more marked than Moja knjiga je u sobi, because it starts with the location, but it is still perfectly everyday Serbian.

A useful way to think about it is:

• Moja knjiga je u sobi. → talking about my book
• U sobi je moja knjiga. → talking about what is in the room or emphasizing the location

So it is not strange at all; it is just a different information structure.

Normally, no.

In a full standard sentence like this, you keep je:

• U sobi je moja knjiga.

Leaving it out would sound incomplete or nonstandard in ordinary speech:

• U sobi moja knjiga → not a normal full sentence

Serbian does sometimes omit forms of to be in certain special contexts, but not in a basic sentence like this one.

There is no separate word, because Serbian has no articles like English a and the.

So soba / sobi can mean:

• a room / in a room
• the room / in the room

The exact meaning comes from context.

In many everyday situations, u sobi will naturally be understood as in the room, but Serbian does not mark that with an article.

Yes, pretty much.

Word-for-word, it is close to:

• U sobi = in the room
• je = is
• moja knjiga = my book

So the literal order is In the room is my book.

That sounds a bit marked in English, but in Serbian this kind of order is very normal because Serbian word order is more flexible and is used to show emphasis and information flow.