Regelheftet er tydelig.

Because -et is the definite ending for many neuter singular nouns in Bokmål.

• et regelhefte = a rule booklet / the rule booklet in indefinite form
• regelheftet = the rule booklet

Unlike English, Norwegian usually adds definiteness to the end of the noun instead of using a separate word like the.

The base noun is hefte, which is a neuter noun:

• et hefte = a booklet / pamphlet

Then it is combined with regel:

• regel = rule
• regelhefte = rule booklet
• regelheftet = the rule booklet

This is a very common Norwegian pattern: nouns are often joined together into one compound word.

Because Norwegian normally writes compound nouns as one word, where English often uses two.

So:

• regel + hefte → regelhefte

This is extremely common in Norwegian. English speakers often want to split compounds, but in Norwegian they are usually written together.

Because Norwegian usually expresses definiteness with a suffix on the noun.

So instead of:

• the rule booklet

Norwegian says:

• regelheftet

The idea of the is already built into the noun, so no extra word is needed here.

Er is the present tense of å være (to be).

So:

• å være = to be
• er = is / am / are, depending on context

In this sentence:

• Regelheftet er tydelig.
• The rule booklet is clear.

Norwegian verbs do not change according to the subject the way English verbs sometimes do.

Here tydelig is used predicatively, meaning it comes after er and describes the subject:

• Regelheftet er tydelig.

In Norwegian, predicative adjectives often agree with gender and number, but some adjectives ending in -ig keep the same form in the neuter singular.

That is why you get:

• en tekst er tydelig
• et regelhefte er tydelig

not tydeligt

So this unchanged form is normal.

Yes. Tydelig often means clear, but depending on context it can also mean things like:

• easy to understand
• distinct
• obvious
• clearly visible/audible

So the exact nuance depends on what is being described. With regelheftet, it usually suggests that the booklet is clearly written, easy to understand, or well presented.

The sentence follows a very basic Norwegian pattern:

• Subject + verb + adjective/complement

So:

• Regelheftet = subject
• er = verb
• tydelig = predicative adjective

This is the normal word order for a simple statement.

Yes, grammatically you can, but the meaning changes slightly.

• Et regelhefte er tydelig = A rule booklet is clear
• Regelheftet er tydelig = The rule booklet is clear

The first is indefinite, so it refers to a rule booklet, not a specific one.
The original sentence refers to a specific booklet.

The plural would be:

• Regelheftene er tydelige. = The rule booklets are clear.

Notice the changes:

• regelheftet → regelheftene
• er stays the same
• tydelig → tydelige

So in the plural, the adjective usually takes -e.

Yes. Norwegian very often builds new nouns by combining smaller nouns into compounds.

Examples of the same pattern:

• skolebok = school book
• arbeidsplass = workplace
• regelhefte = rule booklet

For learners, this is important because long Norwegian words are often just smaller familiar words joined together.