Taj je skup, ali ovaj nije.

They are demonstratives:

• taj = that one
• ovaj = this one

In this sentence, they stand on their own, so they mean that one / this one, not that + noun / this + noun.

So:

• Taj je skup = That one is expensive
• ali ovaj nije = but this one isn’t

Because Serbian, like English, can leave the noun out when it is obvious from context.

For example, if you are looking at two phones, books, coats, etc., you can simply say:

• Taj je skup, ali ovaj nije.

The missing noun is understood. In English we do the same with that one and this one.

Because je is a clitic in Serbian. Clitics usually go in the second position in the clause.

So:

• Taj je skup. ✓

Here, taj takes the first position, and je comes right after it.

This is very normal Serbian word order. English speakers often expect the verb to behave more like English is, but Serbian clitics follow different placement rules.

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

So here it means is:

• Taj je skup = That one is expensive

It agrees with a singular subject.

Nije means is not / isn’t.

It is the negative form of je:

• je = is
• nije = is not

Historically it comes from ne + je, but in standard Serbian it is used as the single form nije.

So:

• ovaj nije = this one isn’t

Because Serbian often omits words that are already clear from context.

So:

• Taj je skup, ali ovaj nije.

literally means:

• That one is expensive, but this one is not.

The adjective skup is understood in the second clause:

• ...ali ovaj nije skup.

Both are correct, but leaving it out sounds natural because the meaning is obvious.

Skup is the masculine singular form of the adjective meaning expensive.

That tells you the implied noun is grammatically masculine singular, or that taj/ovaj are being treated as masculine singular.

Compare:

• masculine singular: skup
• feminine singular: skupa
• neuter singular: skupo
• masculine personal plural: skupi
• other plural forms: skupe

So this sentence is built around a masculine singular referent.

Yes. They change for gender, number, and case.

In this sentence you have the masculine singular nominative forms:

• taj
• ovaj

For comparison:

• feminine: ta, ova
• neuter: to, ovo
• plural: ti / ovi, te / ove, etc., depending on gender and case

So if the thing were feminine, you would say:

• Ta je skupa, ali ova nije.

Very roughly:

• ovaj = something near the speaker = this
• taj = something farther away / that one, often near the listener or already identified

In real usage, demonstratives can depend on context, gesture, and what has already been mentioned. But for a learner, ovaj = this and taj = that is the right starting point.

Yes. They can be either:

1. demonstrative adjectives before a noun

   • taj auto = that car
   • ovaj kaput = this coat

2. pronouns standing alone

   • Taj je skup.
   • Ovaj nije.

In your sentence, they are being used pronominally because the noun is omitted.

Ali means but and clearly marks a contrast.

• Taj je skup, ali ovaj nije. = That one is expensive, but this one isn’t.

You may also hear a in similar contrasts:

• Taj je skup, a ovaj nije.

Both are possible, but:

• ali often sounds like a stronger but
• a often sounds like a softer contrast, something like while / and on the other hand

In this sentence, ali is perfectly natural.

You would change both the demonstrative and the adjective to match the gender.

Feminine singular:

• Ta je skupa, ali ova nije.

Neuter singular:

• To je skupo, ali ovo nije.

So the sentence you were given is specifically the masculine singular version.

Yes, absolutely.

Both are correct:

• Taj je skup, ali ovaj nije.
• Taj je skup, ali ovaj nije skup.

The shorter version is more natural when the adjective is already understood. The longer version is more explicit.