U ovoj rečenici veznik je izostavljen.

Because after u meaning in, Croatian normally uses the locative case when it expresses location, not movement.

So:

• ova rečenica = this sentence (nominative, dictionary/basic form)
• u ovoj rečenici = in this sentence (locative)

Both words change because the adjective/demonstrative ovaj/ova/ovo must agree with the noun rečenica in gender, number, and case.

It is the locative singular.

Why?

• u
  • location = usually locative
• rečenica is feminine singular
• so both words appear in the feminine singular locative:
  • ovoj
  • rečenici

This is very common in Croatian:

• u kući = in the house
• u školi = in school
• u ovoj rečenici = in this sentence

A useful contrast:

• u školi = in school (location, locative)
• u školu = into the school / to school (movement, accusative)

Because rečenica is being put into the locative singular.

The noun rečenica is a feminine noun ending in -a, and many such nouns change like this:

• nominative: rečenica
• locative: rečenici

Similar examples:

• knjiga → u knjizi
• škola → u školi
• ulica → u ulici

So rečenici is simply the case form required by u in this sentence.

Veznik is the grammatical term for a conjunction.

That means a word like:

• i = and
• ali = but
• jer = because
• da = that / so that

So in this sentence, veznik is not just any word; it specifically means a word that connects words, phrases, or clauses.

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

Here it helps form a passive/resultative-style construction with izostavljen:

• veznik je izostavljen

This corresponds to English the conjunction is omitted or has been omitted, depending on context.

So:

• veznik = the conjunction
• je = is
• izostavljen = omitted / left out

Izostavljen is a passive past participle used adjectivally.

It agrees with veznik:

• veznik is masculine singular
• so the form is izostavljen (masculine singular)

Other agreement forms would be different:

• masculine singular: izostavljen
• feminine singular: izostavljena
• neuter singular: izostavljeno
• plural masculine: izostavljeni

For example:

• Veznik je izostavljen.
• Riječ je izostavljena.
• Slovo je izostavljeno.

Croatian word order is more flexible than English word order.

This sentence starts with U ovoj rečenici to set the context first: In this sentence...

Then comes:

• veznik je izostavljen

This is completely natural. You could also say:

• Veznik je izostavljen u ovoj rečenici.

That version is also grammatical, but it places the focus slightly differently. The original sounds like it first establishes the location/context, then gives the information.

Croatian has a tendency for certain short unstressed words, called clitics, to appear near the second position in a clause.

Je is one of these clitics.

In:

• U ovoj rečenici veznik je izostavljen.

the clitic je comes after the first larger phrase before izostavljen. Learners often notice that it does not always stand exactly where English is would stand.

You will see similar patterns elsewhere:

• On je došao.
• U školi je danas hladno.
• Ta knjiga mi se sviđa.

So the placement of je is part of normal Croatian clitic behavior.

Because Croatian does not have articles like English a/an/the.

So veznik can mean:

• a conjunction
• the conjunction

The exact meaning depends on context. In this sentence, English naturally uses the conjunction, but Croatian does not need a separate word for that.

Yes. Depending on context, izostavljen can be translated as:

• omitted
• left out
• sometimes missing or excluded

In a grammar sentence like this one, omitted is probably the most natural English choice.

So the sentence could be understood as:

• In this sentence, the conjunction is omitted.
• In this sentence, the conjunction has been left out.

Yes, effectively it is.

The focus is on the conjunction as the thing affected by the action, not on who omitted it. Croatian often uses this kind of structure when the doer is unimportant or unknown.

Compare:

• active idea: Someone omitted the conjunction.
• Croatian sentence: Veznik je izostavljen. = The conjunction was/is omitted.

That is why the sentence sounds impersonal and descriptive, which fits grammar explanations very well.