Forbrytelsen er alvorlig.

Because -en is the definite singular ending in Norwegian. Instead of putting a separate word like the before the noun, Norwegian usually attaches the definite article to the end of the noun.

• en forbrytelse = an अपराध / a crime
• forbrytelsen = the crime

So forbrytelsen is the definite form.

In Norwegian, the definite article is often built into the noun itself.

So where English says the crime, Norwegian says forbrytelsen.

This is very common in Bokmål:

• en bil → bilen
• et hus → huset
• en forbrytelse → forbrytelsen

If you put an adjective before the noun, then Norwegian often uses both:

• den alvorlige forbrytelsen = the serious crime

The dictionary form is forbrytelse.

A useful set of forms is:

• en forbrytelse = an/the crime in indefinite singular
• forbrytelsen = the crime
• forbrytelser = crimes
• forbrytelsene = the crimes

So if you look the word up, you would usually search for forbrytelse, not forbrytelsen.

Er is the present tense of å være, which means to be.

So:

• jeg er = I am
• du er = you are
• han/hun er = he/she is

In this sentence, er links the subject forbrytelsen with the description alvorlig.

Because here the adjective comes after er, so it is being used predicatively.

After er, Norwegian uses the adjective form that agrees with the subject, but not the same way as in phrases like den alvorlige forbrytelsen.

Compare:

• Forbrytelsen er alvorlig. = The crime is serious.
• Den alvorlige forbrytelsen = The serious crime

So:

• after er: alvorlig
• before a definite noun: alvorlige

Because in Norwegian, definiteness is usually shown on the noun or with a determiner, not on a predicate adjective after er.

That means:

• forbrytelsen er alvorlig = definite noun + predicate adjective
• den alvorlige forbrytelsen = definite noun phrase with adjective before the noun

So even though forbrytelsen is definite, the adjective after er does not become alvorlige just because of that.

It is singular.

You can tell because:

• forbrytelsen = the crime
• forbrytelsene = the crimes

So this sentence talks about one specific crime, not several.

Yes, but it means something different.

• Forbrytelsen er alvorlig = a specific crime is serious
• En forbrytelse er alvorlig = a crime is serious / any crime is serious

The first one points to a particular crime already known from the context. The second is more general or indefinite.

This is normal Norwegian statement word order:

Subject + verb + complement

• Forbrytelsen = subject
• er = verb
• alvorlig = complement/adjective

This is very similar to English:

• The crime is serious

If you make it a yes/no question, the verb comes first:

• Er forbrytelsen alvorlig? = Is the crime serious?

Then Norwegian would usually use:

den alvorlige forbrytelsen

This is a very useful contrast:

• Forbrytelsen er alvorlig. = The crime is serious.
• Den alvorlige forbrytelsen = The serious crime.

So if the adjective is after er, you get alvorlig.
If the adjective is before a definite noun, you get den alvorlige forbrytelsen.

Alvorlig is a very common adjective meaning things like serious, severe, or grave, depending on context.

For example:

• en alvorlig feil = a serious mistake
• et alvorlig problem = a serious problem
• situasjonen er alvorlig = the situation is serious

So in this sentence it describes the crime as serious in nature or importance.