Ovaj telefon je skup, ali ova knjiga nije skupa.

Because Serbian words must agree in gender.

• telefon is masculine
• knjiga is feminine

The demonstrative this changes form to match the noun:

• ovaj telefon = this phone
• ova knjiga = this book

So ovaj is the masculine form, and ova is the feminine form.

For the same reason: adjectives also agree with the noun in gender.

• telefon is masculine, so: telefon je skup
• knjiga is feminine, so: knjiga je skupa

So the adjective skup changes depending on what it describes:

• masculine: skup
• feminine: skupa
• neuter: skupo

Even though the adjective comes after je / nije, it still agrees with the subject.

Here, skup / skupa means expensive.

So:

• Ovaj telefon je skup = This phone is expensive
• ova knjiga nije skupa = this book is not expensive

In other contexts, skup can be related to the idea of something costing a lot, and the noun skup can also mean gathering/meeting, but in this sentence it is clearly the adjective expensive.

Because in Serbian, the negative present tense of biti (to be) has special fused forms.

So instead of ne je, Serbian uses:

• nisam = I am not
• nisi = you are not
• nije = he/she/it is not
• nismo = we are not
• niste = you are not
• nisu = they are not

That is why the sentence says:

• ova knjiga nije skupa = this book is not expensive

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

So:

• telefon je skup = the phone is expensive
• knjiga nije skupa = the book is not expensive

Unlike some other Slavic languages, Serbian normally does use the present-tense form of to be in sentences like this.

Ali means but.

It connects two contrasting ideas:

• Ovaj telefon je skup = This phone is expensive
• ali ova knjiga nije skupa = but this book is not expensive

So the whole sentence contrasts the phone with the book.

Because Serbian does not have articles like English a/an and the.

Instead, meaning comes from context or from words like demonstratives:

• telefon can mean a phone or the phone, depending on context
• ovaj telefon specifically means this phone
• ova knjiga specifically means this book

So Serbian does not need separate article words.

They are in the nominative case.

Why?

Because telefon and knjiga are the subjects of their clauses:

• Ovaj telefon je skup
• ova knjiga nije skupa

The demonstratives and adjectives also match that nominative form:

• ovaj telefon
• ova knjiga

This is the basic dictionary form you usually learn first.

Yes, Serbian word order is more flexible than English, but the sentence you have is the most neutral and natural version for a beginner.

Standard order here is:

• Ovaj telefon je skup, ali ova knjiga nije skupa.

You may hear other orders for emphasis, but they can sound more marked. For example, moving skup or skupa can add focus or contrast. As a learner, it is best to stick with the basic pattern:

subject + je/nije + adjective

Normally, no.

In standard Serbian, you usually keep the verb je in this kind of sentence:

• Ovaj telefon je skup.

Leaving it out would sound incomplete in normal standard Serbian.

Because the adjective must still agree with knjiga, which is feminine.

• knjiga = feminine
• therefore: skupa

So:

• ova knjiga nije skupa = correct
• ova knjiga nije skup = incorrect

Negation does not stop agreement. The adjective still matches the noun in gender and number.

Yes.

• ovaj = this
• telefon = phone

And likewise:

• ova = this
• knjiga = book

So the structure is very similar to English, except that Serbian changes the form of this depending on the gender of the noun.

Often, the ending gives you a clue:

• many feminine nouns end in -a, like knjiga
• many masculine nouns have no ending in the nominative singular, like telefon

So:

• telefon → masculine
• knjiga → feminine

This matters because both the demonstrative and the adjective must agree with the noun:

• ovaj telefon je skup
• ova knjiga nije skupa