Ovaj oglasnik je zanimljiv.

Ovaj means this.

In Ovaj oglasnik je zanimljiv, it matches the noun oglasnik in:

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

So ovaj is the masculine singular nominative form of this.

Compare:

• ovaj oglasnik = this classified ads paper/site
• ova knjiga = this book
• ovo selo = this village

Oglasnik comes from oglas = ad, advertisement, notice.

Oglasnik usually means something like:

• a classified ads paper
• an advertising bulletin
• a listings site/publication

Depending on context, it could refer to:

• a printed classifieds paper
• an online classifieds website
• a publication full of ads

So it is not just any ad, but a place or medium where ads are collected.

In Serbian, every noun has a grammatical gender: masculine, feminine, or neuter.

Oglasnik is masculine because it belongs to a common masculine noun pattern, and nouns ending in a consonant are often masculine.

Since oglasnik is masculine singular, other words agreeing with it also take masculine singular forms:

• ovaj
• zanimljiv

That is why you get:

• ovaj oglasnik
• je zanimljiv

not ova oglasnik or zanimljiva.

Je means is.

It is the 3rd person singular present tense of the verb biti = to be.

So:

• ja sam = I am
• ti si = you are
• on/ona/ono je = he/she/it is

In this sentence:

• Ovaj oglasnik je zanimljiv.
• This classified ads paper/site is interesting.

Zanimljiv means interesting.

It changes form to agree with the noun it describes. Since oglasnik is:

• masculine
• singular
• nominative

the adjective must also be:

• masculine
• singular
• nominative

So you get zanimljiv.

Compare:

• ovaj oglasnik je zanimljiv = this ads paper/site is interesting
• ova knjiga je zanimljiva = this book is interesting
• ovo mesto je zanimljivo = this place is interesting

Serbian does not use articles like English a/an/the.

So a sentence like:

• Oglasnik je zanimljiv

could mean:

• The classifieds paper is interesting
• A classifieds paper is interesting

The exact meaning depends on context.

In your sentence, ovaj already makes it definite:

• ovaj oglasnik = this classifieds paper/site

So no article is needed.

The most neutral word order here is:

• Ovaj oglasnik je zanimljiv.

This is the standard, natural order for a simple statement.

Serbian word order is more flexible than English, though changes often affect emphasis.

For example:

• Zanimljiv je ovaj oglasnik.
  = This classifieds paper/site is interesting with extra emphasis on interesting
• Ovaj oglasnik zanimljiv je.
  = possible, but sounds marked or literary

One important point: je is a clitic, so it usually cannot stand alone in just any position. Learners often notice that these short forms tend to appear near the beginning of the clause.

A simple learner-friendly pronunciation guide is:

• ovaj ≈ OH-vai
• oglasnik ≈ OH-glahs-neek
• je ≈ yeh
• zanimljiv ≈ zah-NEEM-lyiv or zah-NEEM-lyeev

A few useful pronunciation notes:

• j in Serbian is like English y in yes
• lj is a single sound, similar to the lli in million for many learners
• Serbian spelling is very phonetic, so words are usually pronounced much as they are written

No. Zanimljiv means interesting, not interested.

This is a very common thing for learners to check.

• zanimljiv = interesting
• zainteresovan / zainteresirana / zainteresovano = interested

So:

• Ovaj oglasnik je zanimljiv.
  = This classifieds paper/site is interesting.
• Ja sam zainteresovan za ovaj oglasnik.
  = I am interested in this classifieds paper/site.

The main noun phrase is in the nominative case:

• ovaj oglasnik

That is because oglasnik is the subject of the sentence.

The adjective zanimljiv is also in nominative because it agrees with the subject after the verb to be.

So all the key descriptive words line up in the same basic form:

• ovaj — masculine singular nominative
• oglasnik — masculine singular nominative
• zanimljiv — masculine singular nominative

You make it negative by changing je to nije:

• Ovaj oglasnik nije zanimljiv.
• This classifieds paper/site is not interesting.

This is the normal way to negate sentences with to be in Serbian.

The demonstrative and adjective would change to agree with the noun.

For example:

Feminine

• Ova knjiga je zanimljiva.
• This book is interesting.

Neuter

• Ovo mesto je zanimljivo.
• This place is interesting.

So the pattern is:

• masculine: ovaj ... zanimljiv
• feminine: ova ... zanimljiva
• neuter: ovo ... zanimljivo

This kind of agreement is one of the most important things to notice in Serbian.

Yes.

• Oglasnik je zanimljiv. = The classifieds paper/site is interesting or A classifieds paper/site is interesting, depending on context.
• Ovaj oglasnik je zanimljiv. = This classifieds paper/site is interesting.

Adding ovaj makes the reference more specific. It points to a particular one:

• this one here
• this one we are talking about

So both are correct, but they do not mean exactly the same thing.