Knjiga koju sada čitam je dobra.

Because koji is a relative pronoun, and it changes form.

Here is what controls it:

• gender and number come from knjiga → feminine singular
• case comes from its role inside the relative clause

In koju sada čitam, the book is the direct object of čitam (I am reading), so Serbian uses the accusative feminine singular form:

• nominative: koja
• accusative: koju

So knjiga koju čitam is literally the book which I am reading.

Knjiga is in the nominative singular because it is the subject of the main clause.

The main clause is:

• Knjiga ... je dobra.

So:

• knjiga = subject
• je dobra = predicate

Even though the sentence also contains a relative clause, the noun knjiga itself is still the subject of the whole sentence.

In Serbian, the present tense often covers both meanings that English separates into:

• I read
• I am reading

So čitam can mean either, depending on context.

Here, the word sada (now) makes it clear that the meaning is ongoing:

• koju sada čitam = that I am reading now

So Serbian does not need a special progressive form like English does.

Because Serbian usually does not need subject pronouns when the verb ending already shows who is doing the action.

• čitam already means I read / I am reading

So ja is unnecessary unless you want emphasis or contrast.

For example:

• Knjiga koju sada čitam je dobra. = neutral
• Knjiga koju ja sada čitam je dobra. = more emphatic, like the book that I am reading now

Sada means now. It helps show that the reading is happening at the present moment.

Yes, you can also say sad. That is a very common shorter form in everyday speech.

So these are both natural:

• Knjiga koju sada čitam je dobra.
• Knjiga koju sad čitam je dobra.

The version with sada may sound a bit more neutral or slightly more formal.

Because Serbian distinguishes aspect.

• čitati = imperfective, focusing on the process or ongoing action
• pročitati = perfective, focusing on completing the action

Here the sentence describes a book you are currently in the middle of reading, so Serbian uses the imperfective:

• koju sada čitam = that I am reading now

If you used pročitam, it would suggest completion, which does not fit well with an action in progress.

Because je is a clitic in Serbian. Clitics usually go in the second position of the clause, not necessarily right after the first single word.

In this sentence, the first unit is the whole subject phrase:

• Knjiga koju sada čitam

Then the clitic je comes after that:

• Knjiga koju sada čitam je dobra.

This placement is normal Serbian word order.

Because the adjective must agree with knjiga.

Knjiga is:

• feminine
• singular
• nominative

So the adjective also takes the feminine singular nominative form:

• masculine: dobar
• feminine: dobra
• neuter: dobro

That is why the sentence has je dobra.

Yes. Serbian word order is more flexible than English word order, although some versions sound more neutral than others.

The given sentence:

• Knjiga koju sada čitam je dobra.

is a natural, neutral way to say it.

Other orders are possible for emphasis, but they may sound more marked or stylistically different. For example, moving sada or emphasizing dobra can change the focus.

So learners should treat the given version as the safest standard pattern.

No. That would be ungrammatical in standard Serbian.

The reason is that the relative pronoun must match its role inside the relative clause. Since the book is the object of čitam, you need the accusative form koju, not the nominative koja.

So:

• koja = subject form
• koju = object form here

Correct:

• Knjiga koju sada čitam je dobra.

In everyday speech, some speakers do use što in relative clauses, but in a sentence like this, standard Serbian strongly prefers koju.

So for learners, the best form is:

• Knjiga koju sada čitam je dobra.

Using koju is clearer because it shows the correct gender, number, and case.

Yes.

• Knjiga koju čitam je dobra.

still naturally means The book I am reading is good, especially from context.

Adding sada simply makes the time reference more explicit and emphasizes that the reading is happening now.