Onaj mi se sviđa, ali ovaj nije skup.

In Serbian, demonstratives like ovaj and onaj can stand on their own, just like English this one and that one.

So:

• onaj = that one
• ovaj = this one

The noun is simply understood from context. For example, the full idea might be:

• Onaj kaput mi se sviđa, ali ovaj kaput nije skup.

But Serbian often leaves the noun out if it is obvious.

Also, ovaj and onaj change for gender, number, and case. Here they are masculine singular nominative, which suggests the omitted noun is masculine singular.

Because sviđati se works differently from voleti.

• voleti = to love / to like in a more direct way
• sviđati se = to be pleasing to

So Onaj mi se sviđa is literally closer to:

• That one is pleasing to me

That is why Serbian uses:

• mi = to me
• se sviđa = is pleasing / appeals

This is one of the most important patterns to learn, because English speakers often want to build it like I like that one, but Serbian structures it differently.

Mi is the unstressed dative form of ja.

Here it means:

• to me

So:

• Onaj mi se sviđa = That one is pleasing to me = I like that one

You may also see the stressed form meni, but that is usually used for emphasis or contrast:

• Meni se onaj sviđa. = I like that one / That one is the one I like

In your sentence, mi is the normal neutral choice.

In this verb, se is just part of the standard expression sviđati se.

So you should learn it as a unit:

• sviđati se = to be pleasing to / to appeal to
• svideti se = perfective partner, often to come to like / to please

Even though se often has reflexive uses in Serbian, here you should not try to translate it word-for-word as himself / itself / oneself. In this sentence, it is simply part of how the verb works.

Because the grammatical subject is onaj = that one, which is third person singular.

So the verb agrees with onaj, not with me.

• Onaj mi se sviđa. = That one pleases me.
• Oni mi se sviđaju. = Those ones please me.

This is another place where English speakers often get confused. In English, we think of I as the subject in I like that one. But with sviđati se, the thing liked behaves as the subject.

Because the negative form of je is nije.

In Serbian, the present tense of biti in negative is written as a single word:

• nisam
• nisi
• nije
• nismo
• niste
• nisu

So:

• ovaj nije skup = this one is not expensive

You do not say ne je.

Here skup is a predicate adjective, and it agrees with the understood noun.

Since ovaj is masculine singular, the adjective is also masculine singular:

• ovaj nije skup = masculine singular

Compare:

• ova nije skupa = feminine singular
• ovo nije skupo = neuter singular
• ovi nisu skupi = masculine plural

So skup is used because the omitted noun is understood as masculine singular.

They are both demonstratives, but they point to different things:

• ovaj = this, something near the speaker
• onaj = that, something farther away

Serbian also has taj, which is often the middle distance word:

• ovaj = this here
• taj = that / that near you / that just mentioned
• onaj = that over there

In everyday speech, the boundaries are not always perfectly strict, but this is the basic idea.

Serbian word order is flexible, but not completely free.

The short words mi and se are clitics, and clitics usually go in the second position of their clause. That is why mi se appears early.

So these are possible, depending on emphasis:

• Onaj mi se sviđa. = neutral, very natural
• Onaj se mi sviđa. = not normal
• Sviđa mi se onaj. = also possible, often with a different emphasis

A good beginner rule is:

• treat mi se as a little clitic bundle that usually comes near the start of the clause

Yes, that is possible.

Both of these are natural:

• Onaj mi se sviđa, ali ovaj nije skup.
• Sviđa mi se onaj, ali ovaj nije skup.

The difference is mostly emphasis and rhythm.

• Onaj mi se sviđa puts onaj up front for contrast: that one, I like
• Sviđa mi se onaj starts with the verb phrase, which can sound a bit more like I like that one

Serbian often changes word order to highlight different parts of the sentence.

Yes, in standard writing you normally put a comma before ali.

Here ali means but, and it links two clauses:

• Onaj mi se sviđa
• ovaj nije skup

So the comma is standard:

• Onaj mi se sviđa, ali ovaj nije skup.

Grammatically, it can refer to either, depending on context.

• Onaj mi se sviđa can mean I like that one about an object
• it can also mean I like that one about a person

But the second part, ovaj nije skup, strongly suggests an object, because skup means expensive.

So in this full sentence, the most natural interpretation is that you are comparing objects for sale, such as coats, phones, or cars.

The demonstratives and the adjective would change to match the gender.

For example:

Feminine singular

• Ona mi se sviđa, ali ova nije skupa.

Neuter singular

• Ono mi se sviđa, ali ovo nije skupo.

So the sentence you have is masculine because of:

• onaj
• ovaj
• skup

All three point to a masculine singular noun being understood.