Handlingen er kriminell.

-en is the definite article suffix in Norwegian for many common-gender nouns. So:

• en handling = an act / an action
• handlingen = the act / the action

Unlike English, Norwegian often puts the at the end of the noun instead of using a separate word.

Handling is usually a common-gender noun in Bokmål, so its forms are:

• en handling = an act/action
• handlingen = the act/action

Yes, gender matters because it affects articles and adjective endings in other situations. In this sentence, the definite form handlingen shows that the noun is common gender.

In Norwegian, definiteness is often shown by adding an ending to the noun:

• en bok = a book
• boken = the book

So handlingen already means the act / the action. You do not need a separate the in a simple sentence like this.

Er means is / am / are. It is the present tense of å være (to be).

So:

• Handlingen er kriminell = The act is criminal

This is a very common sentence pattern in Norwegian:

• noun + er + adjective

Examples:

• Bilen er dyr = The car is expensive
• Huset er stort = The house is big

The adjective form depends on what it describes.

Here, handlingen is a singular common-gender noun, so the basic adjective form is used:

• en kriminell handling = a criminal act
• Handlingen er kriminell = The act is criminal

Other forms:

• et kriminelt forhold = a criminal matter/circumstance (neuter singular)
• kriminelle handlinger = criminal acts (plural)

So kriminell is correct because it matches a singular common-gender noun.

Here it is an adjective, describing handlingen.

• Handlingen er kriminell = The act is criminal

But kriminell can also be a noun in other contexts:

• Han er en kriminell = He is a criminal

So the word can have more than one role, but in this sentence it is definitely an adjective.

Yes. Handling can have different meanings depending on context, including:

• act / action / deed
• plot (of a book, film, etc.)

So handlingen could also mean the plot in some contexts. However, in the sentence Handlingen er kriminell, the natural meaning is the act/action is criminal, not the plot is criminal, unless the context clearly points to a story.

Yes. This is standard Norwegian word order for a simple statement:

• Subject + verb + complement
• Handlingen
  • er
    • kriminell

This is very similar to English:

• The act
  • is
    • criminal

Yes, but it means something slightly different.

• Handlingen er kriminell = The act is criminal
• Den handlingen er kriminell = That act is criminal

Adding den makes it more specifically demonstrative, like that in English.

Yes, grammatically you can, but it sounds more repetitive:

• Handlingen er kriminell = The act is criminal
• Handlingen er en kriminell handling = The act is a criminal act

The second version is possible, but it repeats the same idea more heavily. The shorter sentence is usually more natural.

A rough guide is:

• kriminell ≈ kree-mee-NELL

The stress is on the last syllable: -nell.

A few pronunciation notes:

• kri sounds roughly like kree
• mi sounds like mee
• nell has the main stress

If you are learning pronunciation, it is best to listen to native audio as well, because Norwegian melody and vowel quality matter a lot.

Not necessarily. In Bokmål, this sentence is:

• Handlingen er kriminell.

In Nynorsk, you might more naturally see forms like:

• Handlinga er kriminell.

The main difference here is the definite ending:

• Bokmål often: -en
• Nynorsk often: -a

But the exact wording can vary depending on style and dialect influence.