Glavno pravilo je važno.

Because pravilo is a neuter singular noun, and adjectives must agree with the noun they describe.

• glavno pravilo = adjective + noun
• pravilo je važno = noun + to be
  • predicate adjective

In both places, the adjective is matching pravilo, so it takes the neuter singular form:

• masculine: glavan / važan or more standard adjective forms depending on the word
• feminine: glavna / važna
• neuter: glavno / važno

So the -o ending here tells you the adjectives are agreeing with a neuter singular noun.

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

So the structure is:

• Glavno pravilo = the main rule
• je = is
• važno = important

This is the normal Serbian equivalent of an English is sentence. Unlike in some other Slavic languages, Serbian normally keeps the present-tense form of to be in standard sentences like this.

They are doing two different jobs.

• glavno is an attributive adjective: it directly modifies the noun pravilo
  • glavno pravilo = main rule
• važno is a predicate adjective: it comes after je and tells you something about the subject
  • pravilo je važno = the rule is important

So the sentence has this structure:

• [adjective + noun] + je + [predicate adjective]

That is very normal in Serbian.

Yes. Pravilo is a neuter noun, and its dictionary form is nominative singular.

A useful clue is that many Serbian nouns ending in -o or -e are neuter, including:

• pravilo = rule
• selo = village
• more = sea

Because pravilo is neuter, adjectives with it must also be in neuter form:

• glavno pravilo
• važno pravilo
• ovo pravilo

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

So pravilo can mean:

• a rule
• the rule

and the exact meaning depends on context.

In Glavno pravilo je važno, English often uses the: The main rule is important, but Serbian does not need a separate word for that.

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

• Glavno pravilo je važno.

You might also hear:

• Važno je glavno pravilo.

But that version sounds more marked and puts stronger focus on važno or changes the information structure.

For a learner, the safest choice is the original order: Glavno pravilo je važno.

You could, but it would mean something different.

• Glavno pravilo je važno. = The main rule is important.
• Pravilo je glavno. = The rule is the main one / The rule is primary.

In the original sentence, glavno belongs with pravilo as part of the noun phrase. In Pravilo je glavno, glavno becomes the predicate adjective, so the meaning shifts.

You negate je by using nije:

• Glavno pravilo nije važno.

That means The main rule is not important.

This is a very common pattern in Serbian:

• On je ovde. → On nije ovde.
• Pravilo je tačno. → Pravilo nije tačno.

Grammatically, važno is describing the subject glavno pravilo, but it does so through the verb je.

So:

• glavno directly modifies pravilo
• važno says something about glavno pravilo as a whole

That is why važno is called a predicate adjective.

A simple way to see it:

• glavno pravilo = one unit
• je važno = what is being said about that unit

Several words show that it is singular:

• pravilo is singular
• je is singular (is)
• glavno is singular neuter
• važno is singular neuter

If it were plural, you would get:

• Glavna pravila su važna. = The main rules are important.

Notice the changes:

• pravilo → pravila
• je → su
• glavno → glavna
• važno → važna