Ovaj telefon je noviji nego onaj.

Because telefon is a masculine singular noun, so the demonstratives have to match it.

• ovaj = this, masculine singular
• onaj = that, masculine singular

If the noun were feminine or neuter, the form would change:

• ova knjiga = this book
• ovo selo = this village

In your sentence, onaj stands for onaj telefon, so it is also masculine singular.

Noviji is the comparative form of nov.

• nov = new
• noviji = newer

So the sentence is using an adjective comparison: one phone is newer than the other.

Also, noviji is in the masculine singular nominative form, because it agrees with telefon.

The comparative of nov is formed as noviji.

This is a common Serbian pattern:

• nov → noviji = new → newer
• star → stariji = old → older
• lep → lepši = beautiful → more beautiful / prettier

There is no single ending that works for every adjective, so comparatives need to be learned pattern by pattern. For nov, the correct comparative is noviji.

In comparisons, Serbian often uses nego to mean than.

So:

• noviji nego onaj = newer than that one

This is a very normal way to compare two things.

Serbian can also use od in many comparisons:

• Ovaj telefon je noviji od onog.

That means the same thing, but the grammar after od is different.

Because nego and od behave differently.

With nego, Serbian often keeps the compared word in the nominative:

• Ovaj telefon je noviji nego onaj.

With od, the following noun or pronoun goes into the genitive:

• Ovaj telefon je noviji od onog.

So both are correct, but the case changes depending on which comparison structure you use.

Yes. Onaj can stand alone because the noun is understood from context.

So:

• Ovaj telefon je noviji nego onaj.

really means:

• Ovaj telefon je noviji nego onaj telefon.

Repeating telefon is possible, but usually unnecessary. Serbian often leaves out a noun when it is already clear.

Yes, in a normal sentence like this, je is necessary.

• je = is

Serbian uses the verb biti = to be in sentences with predicate adjectives:

• Telefon je nov. = The phone is new.
• Telefon je noviji. = The phone is newer.

So Ovaj telefon noviji nego onaj would sound incomplete in standard Serbian.

Telefon is in the nominative singular.

It is the subject of the sentence:

• Ovaj telefon = this phone

The adjective noviji also matches that nominative masculine singular form.

Yes, absolutely. That is grammatical.

But in everyday Serbian, speakers usually prefer the shorter version:

• Ovaj telefon je noviji nego onaj.

Once telefon has already been mentioned, repeating it is often unnecessary.

These are all demonstratives, but they point to different degrees of distance.

A simple way to think of them is:

• ovaj = this one, near me
• taj = that one, near you / already mentioned
• onaj = that one over there / farther away

In your sentence, ovaj telefon and onaj create a clear contrast: this phone versus that one.

In real speech, the exact difference between taj and onaj can depend on context, but this basic distinction is a good starting point.

Yes, Serbian word order is fairly flexible, but the version you have is the most neutral and natural:

• Ovaj telefon je noviji nego onaj.

Other orders are possible for emphasis, but they sound more marked, for example:

• Noviji je ovaj telefon nego onaj.

That puts more focus on noviji.

For learners, the original order is the safest choice.