Ona čita knjigu u sobi.

Because knjiga is the dictionary form, or nominative form, meaning book.

In this sentence, book is the direct object of čita (reads), so Serbian uses the accusative case. For a feminine singular noun like knjiga, the accusative ending is -u:

• knjiga = nominative
• knjigu = accusative

So:

• Ona čita knjigu. = She is reading a/the book.

Because after u meaning in, Serbian uses the locative case when talking about a location.

So:

• soba = room (nominative)
• u sobi = in the room (locative)

This is a very common pattern:

• u školi = in school
• u kući = in the house
• u sobi = in the room

Ona is in the nominative case because it is the subject of the sentence.

The subject is the person doing the action, and in Serbian the subject normally appears in the nominative:

• ona = she
• on = he
• ja = I

So here:

• Ona = the one who is reading

Yes, very often you can.

Serbian usually allows the subject pronoun to be omitted when the verb already makes the person clear. Since čita means he/she/it reads/is reading, the sentence can simply be:

• Čita knjigu u sobi.

This still means She is reading a/the book in the room, though without context it could also mean He is reading... because the present-tense verb form čita does not show gender.

So ona is often included only when you want to:

• clarify who is doing the action
• emphasize she
• contrast with someone else

It can mean either one, depending on context.

Serbian present tense often covers both:

• She reads
• She is reading

So Ona čita knjigu u sobi could mean:

• she is reading a book right now in the room, or
• she reads a book in the room as a regular/habitual action

The wider context tells you which meaning is intended.

Čita is:

• present tense
• 3rd person singular
• from the verb čitati = to read
• imperfective aspect

Examples of the present tense:

• ja čitam = I read / am reading
• ti čitaš = you read / are reading
• on/ona čita = he/she reads / is reading

Because it is imperfective, it describes an ongoing, repeated, or general action rather than a completed one.

Because Serbian has no articles.

English uses:

• a book
• the book

Serbian usually just says:

• knjiga / knjigu

Context tells you whether it means a book, the book, or sometimes just book in a more general sense.

So knjigu could mean:

• a book
• the book

depending on the situation.

Yes. Serbian word order is much freer than English word order because the noun endings show the grammatical roles.

The neutral order here is:

• Ona čita knjigu u sobi.

But you may also hear:

• U sobi ona čita knjigu. = In the room, she is reading a book.
• Knjigu ona čita u sobi. = emphasis on the book
• Ona u sobi čita knjigu. = also natural

The meaning stays basically the same, but the focus or emphasis changes.

This is a very important Serbian pattern.

• u sobi = in the room → location, no movement → locative
• u sobu = into the room → direction/movement → accusative

Compare:

• Ona čita knjigu u sobi. = She is reading a book in the room.
• Ona ulazi u sobu. = She is going into the room.

So after u, the case depends on meaning:

• where? → locative
• where to? → accusative

The letter č is pronounced like ch in church, but usually a bit firmer and more distinct.

So:

• čita sounds roughly like CHEE-tah
• knjigu is roughly KNYEE-goo
• sobi is roughly SOH-bee

Also, č and ć are different letters in Serbian, and learners often confuse them. In this sentence, it is č.

In Serbian Cyrillic, the sentence is:

• Она чита књигу у соби.

Letter by letter:

• Ona → Она
• čita → чита
• knjigu → књигу
• u sobi → у соби

Serbian is commonly written in both Latin and Cyrillic, and both are standard.

You add ne before the verb:

• Ona ne čita knjigu u sobi.

This means:

• She is not reading a/the book in the room.

This is the normal way to negate present-tense verbs in Serbian.

Usually yes, when referring to a person.

• ona = she

It is the feminine singular pronoun. But Serbian also uses grammatical gender for nouns, so ona can sometimes refer to a feminine noun that is not a person, depending on context.

In this sentence, though, a learner will normally understand ona as she.