Han er interessert i musikk.

Because interessert is used like an adjective here, not like a main verb.

So Norwegian says:

• Han er interessert i musikk = He is interested in music

This works like English:

• He is tired
• He is happy
• He is interested in music

Even though interessert originally comes from a past participle, in this sentence it functions as an adjective, so it goes with er.

No. Interessert means interested, not interesting.

• interessert = interested
• interessant = interesting

So:

• Han er interessert i musikk = He is interested in music
• Musikk er interessant = Music is interesting

This is a very common learner mix-up.

Because Norwegian uses the fixed expression interessert i for interested in.

So the pattern is:

• å være interessert i noe = to be interested in something

Examples:

• Jeg er interessert i sport = I am interested in sports
• Hun er interessert i språk = She is interested in languages

You usually cannot leave out i here.

Yes, i often means in, but in this sentence it is best understood as part of the fixed phrase interessert i.

So although English also says interested in, it is more useful to learn this as a whole pattern rather than translating i word by word every time.

Because musikk is being used in a general sense, like music in English.

Compare:

• Han er interessert i musikk = He is interested in music
• Han er interessert i klassisk musikk = He is interested in classical music

You would not normally say i en musikk here.

In Norwegian, mass nouns and general categories often appear without an article, just as in English.

It is a noun.

More specifically, it is an uncountable noun in this sentence, used in a general sense. That is why it appears without an article.

• musikk = music

It is the thing he is interested in.

No, this is normal basic Norwegian word order.

The sentence is:

• Han = subject
• er = verb
• interessert i musikk = description/complement

So it follows the usual pattern:

Subject + verb + rest

Because this is a simple main clause, it looks very similar to English.

You just change the subject and the form of to be if needed.

• Han er interessert i musikk = He is interested in music
• Hun er interessert i musikk = She is interested in music
• Jeg er interessert i musikk = I am interested in music
• Vi er interessert i musikk = We are interested in music

Notice that er stays the same for most subjects in the present tense.

Put ikke after the verb:

• Han er ikke interessert i musikk = He is not interested in music

This is standard Norwegian sentence negation in a simple main clause.

Pattern:

• Han er ...
• Han er ikke ...

In a yes/no question, Norwegian usually puts the verb first:

• Er han interessert i musikk? = Is he interested in music?

This is very common and works much like English inversion:

• He is interested ...
• Is he interested ...?

A careful approximate pronunciation is:

• interessert ≈ in-te-re-SERT

The stress is near the end, on -sert.

A few useful points:

• The r is Norwegian, so its exact sound depends on dialect.
• The final -t may be weak or not strongly released in natural speech.
• Do not pronounce it exactly like the English word interested.

If you want to sound natural, it helps to learn the whole phrase together:

• er interessert i

Not necessarily. By itself, it simply means he is interested in music.

It tells you about his interest, not whether he:

• plays music,
• studies music,
• works in music,
• or just likes listening to it.

If you want to say he works with music or studies it, Norwegian would usually say that more directly, for example:

• Han driver med musikk = He does music / He is involved in music
• Han studerer musikk = He studies music

So interessert i musikk is broader and less specific.

Yes, absolutely.

• Han er veldig interessert i musikk = He is very interested in music

You can also use other adverbs:

• Han er ganske interessert i musikk = He is quite interested in music
• Han er spesielt interessert i musikk = He is especially interested in music

So interessert behaves like an adjective that can be modified by words like veldig.