Čudovište je opasno.

Because čudovište is a neuter singular noun, and the adjective has to agree with it.

The adjective opasan means dangerous, and its basic gender forms are:

• opasan = masculine
• opasna = feminine
• opasno = neuter

So:

• opasan pas = a dangerous dog
• opasna zmija = a dangerous snake
• opasno čudovište = a dangerous monster

In Čudovište je opasno, the adjective is used after je (is), but it still agrees with čudovište, so it stays neuter singular: opasno.

Čudovište is neuter.

A useful clue is that many Croatian neuter nouns end in -o or -e, and čudovište ends in -e. So learners often recognize it as neuter from the ending.

That is why the sentence uses:

• je = is
• opasno = neuter form of dangerous

So the full structure is:

• čudovište = the monster
• je = is
• opasno = dangerous

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

So:

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

In this sentence, čudovište is singular, so Croatian uses je:

• Čudovište je opasno. = The monster is dangerous.

Because Croatian does not have articles like English a and the.

So čudovište can mean:

• a monster
• the monster

Which one is meant depends on the context.

That means Čudovište je opasno can be understood as:

• The monster is dangerous
• or A monster is dangerous

In many learning materials, the context or translation chooses one of those.

Yes. Čudovište is in the nominative singular because it is the subject of the sentence.

This is a basic subject + verb + adjective sentence:

• Čudovište = subject
• je = verb
• opasno = predicate adjective

After biti (to be), Croatian normally uses the adjective in the nominative as well, agreeing with the subject.

So both the noun and the adjective are in forms that match a neuter singular subject.

Yes, Croatian word order is more flexible than English word order.

The neutral, most straightforward order is:

• Čudovište je opasno.

But Croatian can sometimes rearrange words for emphasis or style. For example:

• Opasno je čudovište.

That sounds more marked and less neutral, something like It is the monster that is dangerous or Dangerous is the monster, depending on context.

For learners, the safest default is:

• subject + je + adjective

So Čudovište je opasno is the most natural basic version.

A rough English-style pronunciation is:

• čudovište ≈ CHOO-do-veesh-teh
• je ≈ yeh
• opasno ≈ oh-PAHS-noh

A few important sounds:

• č sounds roughly like ch in chocolate, but a bit firmer
• š sounds like sh in ship
• j sounds like y in yes

So the whole sentence sounds roughly like:

• CHOO-do-veesh-teh yeh oh-PAHS-noh

Normally, no. In Croatian, the present tense of to be is usually expressed, so you say:

• Čudovište je opasno.

not normally:

• Čudovište opasno.

Leaving out je would sound incomplete in standard Croatian.

So unlike some languages where is can be omitted in the present tense, Croatian generally keeps it.

No. Here opasno is an adjective, not an adverb.

It may look confusing because in Croatian some adjective forms and adverbs can look similar. But in this sentence, opasno describes čudovište, so it is an adjective agreeing with a neuter singular noun.

Compare:

• Čudovište je opasno. = The monster is dangerous.
  (opasno = adjective)

• Vozi opasno. = He/She drives dangerously.
  (opasno = adverb)

So in your sentence, it means dangerous, not dangerously.