Tražim knjigu, ali je ne vidim.

Because knjigu is the accusative singular form of knjiga.

In this sentence, knjigu is the direct object of tražim (I am looking for), so Serbian uses the accusative case.

• nominative: knjiga = book
• accusative: knjigu = book as the object

For many feminine nouns ending in -a, the accusative singular ends in -u.

It can mean a book or the book. Serbian does not have articles like English a/an/the.

So the exact meaning depends on context:

• Tražim knjigu could mean I’m looking for a book
• or I’m looking for the book

The rest of the situation tells you which one is meant.

Tražim is the 1st person singular present tense of tražiti.

It means:

• I look for
• I am looking for
• sometimes I’m searching for

In Serbian, the present tense often covers both the English simple present and present continuous, so tražim can naturally mean I’m looking for here.

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

Tražim already means I am looking for because the ending -m marks I.

So:

• Tražim knjigu = I’m looking for a/the book
• Ja tražim knjigu also works, but ja adds emphasis, contrast, or clarity

For example, Ja tražim knjigu, a on traži svesku = I’m looking for the book, and he’s looking for the notebook.

No. Here je is not the verb is.

In this sentence, je is an unstressed object pronoun meaning it (literally feminine her/it, because knjiga is feminine in Serbian).

So:

• vidim = I see
• je vidim = I see it / her

The form je can also mean is in other sentences, but here the context shows it is a pronoun.

Because Serbian pronouns follow grammatical gender, not natural gender.

The noun knjiga is feminine, so when you refer back to it with a pronoun, you use a feminine form:

• knjiga → je

Even though English uses it for objects, Serbian still treats the noun as feminine grammatically.

So je here means it, referring to knjiga.

Ne is the word that makes the verb negative.

• vidim = I see
• ne vidim = I do not see / I don’t see

So in ali je ne vidim, the ne is simply negating vidim.

Because je is an unstressed clitic pronoun, and clitics in Serbian usually stand near the beginning of the clause.

So this order is natural:

• ali je ne vidim

Here is the rough structure:

• ali = but
• je = it
• ne vidim = I don’t see

A very literal word-for-word layout would look like:

• but it not see-I

That sounds strange in English, but it is normal Serbian structure.

If you want a stronger, stressed form of the pronoun, Serbian can use nju instead:

• ali nju ne vidim = but I don’t see it/her

That version gives the pronoun more emphasis.

Yes, you may hear ju as a feminine accusative pronoun, especially in everyday speech.

So both of these may be encountered:

• ali je ne vidim
• ali ju ne vidim

The sentence you were given uses je, which is perfectly normal. Learners should mainly recognize that both forms can refer back to a feminine noun like knjiga.

You can sometimes omit the object if it is understood from context, but je makes it explicit: I don’t see it.

So:

• Tražim knjigu, ali je ne vidim = I’m looking for the book, but I don’t see it
• Tražim knjigu, ali ne vidim sounds incomplete unless the missing object is very clear from the situation

Including je is the most natural way to refer back to knjigu.

Because videti / vidim means to see, while gledati / gledam means to look at / watch.

That distinction matters:

• tražim = I’m looking for
• vidim = I see
• gledam = I’m looking at / watching

So the sentence means:

• I’m looking for the book, but I don’t see it

not

• I’m looking for the book, but I’m not watching it

Because ali means but, and when it joins two clauses in Serbian, a comma is normally used before it.

So:

• Tražim knjigu, ali je ne vidim.

This is standard Serbian punctuation.