Ovaj telefon je skup, ali on nije dobar.

Serbian does not have articles like the and a/an. So telefon can mean phone, a phone, or the phone, depending on context.

In this sentence, ovaj already makes it specific, so ovaj telefon means this phone.

Ovaj means this.

It appears as ovaj because it must agree with telefon in:

• gender: masculine
• number: singular
• case: nominative

Since telefon is a masculine singular noun and it is the subject here, ovaj is the matching form.

Because Serbian normally uses the verb biti (to be) in sentences like this.

So:

• je = is
• ovaj telefon je skup = this phone is expensive

Unlike some other Slavic languages, Serbian usually keeps the present-tense copula in standard sentences of this type.

Both adjectives agree with the thing being described, which is telefon.

Since telefon is:

• masculine
• singular
• nominative

the adjectives are also masculine singular nominative:

• skup = expensive
• dobar = good

So the grammar is:

• telefon je skup
• telefon nije dobar

After je / nije, Serbian normally uses the predicate form skup.

So:

• Ovaj telefon je skup. = correct
• Ovaj skupi telefon... = also correct, but here skupi is used before the noun, not after je

A useful contrast is:

• ovaj skupi telefon = this expensive phone
• ovaj telefon je skup = this phone is expensive

Because Serbian pronouns follow grammatical gender.

The noun telefon is masculine, so the matching pronoun is on.

So although English uses it for things, Serbian uses:

• on for masculine nouns
• ona for feminine nouns
• ono for neuter nouns

Here, on refers back to telefon.

Yes, absolutely. That is very natural Serbian.

Serbian often omits subject pronouns when they are already clear from context. So both are possible:

• Ovaj telefon je skup, ali on nije dobar.
• Ovaj telefon je skup, ali nije dobar.

Including on adds a bit more emphasis or contrast, something like but it / that one is not good.

Because the negative present forms of biti (to be) are written as single words in Serbian:

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

So nije dobar means is not good.

Because je is a clitic in Serbian, and clitics usually go in the second position of the clause.

In:

• Ovaj telefon je skup

the first unit is ovaj telefon, and then the clitic je comes after it.

This is why Serbian word order may look a little different from English. You should get used to je appearing early in the clause.

Ali means but and shows a strong contrast.

So:

• Ovaj telefon je skup, ali on nije dobar. means there is a clear opposition between expensive and not good.

You may also see a, which can also link contrasting ideas, but ali is the more direct equivalent of English but.