Opomena je nova.

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

So:

• opomena = warning / reprimand
• je = is
• nova = new

That makes Opomena je nova. literally The warning is new.

Because nova has to agree with opomena in gender, number, and case.

• opomena is feminine singular
• so the adjective must also be feminine singular
• the feminine singular form of nov is nova

Compare:

• nov = masculine singular
• nova = feminine singular
• novo = neuter singular

So nova is used because opomena is feminine.

In Serbian, many nouns ending in -a in the dictionary form are feminine.

So opomena is a feminine noun.

That is why it takes:

• nova, not nov
• and other feminine forms in the rest of the sentence as needed

This is grammatical gender, not biological sex.

It is in the nominative singular.

Why?

• It is the subject of the sentence.
• With biti (to be), the subject stays in the nominative.
• The predicate adjective nova also appears in a matching form.

So the basic structure is:

• Opomena = subject, nominative singular
• je nova = predicate, is new

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

So Opomena je nova. can mean:

• The warning is new
• A warning is new

Which one sounds best depends on context. In real use, the situation usually makes it clear.

Because this is a full sentence, not just a noun phrase.

• nova opomena = a new warning / the new warning
  This is a noun phrase.
• Opomena je nova. = The warning is new.
  This is a complete sentence.

In your sentence, nova is not directly describing the noun inside a phrase. It is a predicate adjective after the verb je.

Yes, Serbian word order is more flexible than English, but the neutral order here is:

• Opomena je nova.

Other orders are possible for emphasis, for example:

• Nova je opomena. = puts more focus on new
• Opomena nova je. = possible in poetry or marked style, but not the normal everyday order

So for a learner, Opomena je nova. is the safest and most natural version.

A simple English-friendly approximation is:

• opomena ≈ o-po-ME-na
• je ≈ ye
• nova ≈ NO-va

So the whole sentence sounds roughly like:

o-po-ME-na ye NO-va

A few useful pronunciation notes:

• Serbian j sounds like English y in yes
• each vowel is pronounced clearly
• r is usually rolled or tapped, though there is no r in this sentence

In standard Serbian, no—you normally keep je in this kind of present-tense sentence.

So you say:

• Opomena je nova.

Not normally:

• Opomena nova.

Leaving out je may appear in very special styles such as headlines, notes, or poetic language, but it is not the normal standard sentence pattern.

It is an adjective.

The basic dictionary form is usually given as nov (new), and then it changes form to match the noun:

• nov = masculine singular
• nova = feminine singular
• novo = neuter singular

In this sentence, nova is functioning as a predicate adjective, because it comes after je and tells you a quality of the subject:

• The warning is new.

They are related, but they are not the same structure.

• Opomena je nova. = The warning is new.
  A complete sentence.
• Nova opomena. = A new warning / the new warning
  Just a noun phrase, not a full sentence by itself.

So if you want to make a statement, use:

• Opomena je nova.

If you only want to name something, use:

• nova opomena