Ovaj trenutak je važan.

Because ovaj has to agree with trenutak.

In Serbian, demonstratives like this change form depending on the noun’s:

• gender
• number
• case

Trenutak is:

• masculine
• singular
• nominative

So the correct form is ovaj.

Compare:

• ovaj trenutak = this moment
• ova knjiga = this book
• ovo vreme = this time

So ovaj is simply the masculine singular nominative form.

Trenutak is in the nominative singular because it is the subject of the sentence.

In Ovaj trenutak je važan, the structure is basically:

• Ovaj trenutak = the subject
• je važan = the predicate

The basic dictionary form of the noun is also trenutak, so this is the normal subject form.

Je is the 3rd person singular present tense of the verb biti, which means to be.

So:

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

Here, trenutak is grammatically singular, so Serbian uses je:

• Ovaj trenutak je važan = This moment is important.

Because važan must agree with trenutak, which is masculine singular.

In Serbian, adjectives change to match the noun’s:

• gender
• number
• sometimes case

Since trenutak is masculine singular, the adjective must also be masculine singular:

• važan = masculine
• važna = feminine
• važno = neuter

Examples:

• važan trenutak = an important moment
• važna odluka = an important decision
• važno pitanje = an important question

In this sentence, it is part of the predicate.

There is a difference between:

• važan trenutak = an important moment
  
  • here the adjective directly modifies the noun
• trenutak je važan = the moment is important
  
  • here the adjective comes after je and functions as a predicate adjective

So in Ovaj trenutak je važan, važan is not just sitting next to the noun as an attribute; it is saying something about the subject through the verb je.

Yes, Serbian word order is more flexible than English, although not every order sounds equally natural in every context.

The most neutral order here is:

• Ovaj trenutak je važan.

But you may also hear:

• Važan je ovaj trenutak.
• Ovaj je trenutak važan.

These alternatives usually add a different emphasis.

For example:

• Važan je ovaj trenutak can emphasize important
• Ovaj je trenutak važan can sound slightly more literary or contrastive

For a learner, the safest and most natural default is the original sentence: Ovaj trenutak je važan.

Because Serbian does not have articles like English.

English distinguishes:

• a moment
• the moment

Serbian usually leaves that distinction to context.

So:

• trenutak can mean a moment or the moment, depending on the situation
• ovaj trenutak already means this moment, so no article is needed anyway

This is very normal in Serbian.

A simple learner-friendly approximation is:

OH-vai TREH-noo-tahk yeh VAH-zhan

A few useful sound notes:

• j is pronounced like English y
  • so je sounds like yeh
• ž sounds like the s in measure
  • so važan has a zh sound
• Serbian pronunciation is generally quite phonetic, so words are usually pronounced close to how they are spelled

Word-by-word:

• ovaj ≈ OH-vai
• trenutak ≈ TREH-noo-tahk
• je ≈ yeh
• važan ≈ VAH-zhan

No. Trenutak is a very common native Serbian word meaning moment, instant, or point in time.

You may also see words like:

• momenat
• sometimes moment

These are also used, but trenutak is very common and feels fully standard and natural.

So Ovaj trenutak je važan is an excellent, natural sentence.

Yes, you could.

Both važan and bitan can mean important.

Very roughly:

• važan = important
• bitan = important, significant, essential, relevant

In many everyday contexts, they overlap a lot.

So these are both possible:

• Ovaj trenutak je važan.
• Ovaj trenutak je bitan.

Sometimes bitan can feel a little more like matters / is significant, but the difference is often small.

In standard Serbian, you normally keep je here.

So the correct full sentence is:

• Ovaj trenutak je važan.

Leaving out je would sound incomplete in standard usage.

Serbian does sometimes omit parts of to be in certain styles or very informal situations, but for a learner, the rule is simple:

• if you want to say X is Y in the present tense, use the correct form of biti
• here that form is je

In this sentence, both are in masculine singular nominative because they relate to trenutak as the subject.

But these words change in other cases.

For example:

• ovaj trenutak = this moment
• ovog trenutka = of this moment / at this moment
• ovom trenutku = to this moment / in this moment

And adjectives change too:

• važan trenutak
• važnog trenutka
• važnom trenutku

So this sentence is a good example of Serbian agreement in its basic nominative form, but the forms will change when the noun changes case.