Innfallet er morsomt.

Innfall usually means a sudden idea, impulse, whim, or notion. It often suggests that the thought came to someone unexpectedly, rather than being carefully planned.

So in this sentence, innfallet is not just any general idea; it is that particular sudden idea/impulse.

Because innfallet is the definite singular form of the noun.

In Norwegian, the definite article is usually attached to the end of the noun:

• et innfall = an idea / an impulse
• innfallet = the idea / the impulse

So the -et ending here means the.

Because innfall is a neuter noun in Norwegian.

For many neuter nouns in Bokmål:

• et
  • noun = indefinite singular
• noun + -et = definite singular

So:

• et innfall
• innfallet

If the noun were a common-gender noun instead, it would usually take -en in the definite singular.

Because the adjective must agree with the noun.

Innfallet is neuter, so the predicative adjective takes the neuter singular form:

• common gender singular: morsom
• neuter singular: morsomt
• plural / definite: morsomme

Since innfallet is neuter singular, the correct form is morsomt.

It is the adjective used after the verb er. This is called a predicative use of the adjective.

Compare:

• et morsomt innfall = a funny idea
  (adjective before the noun)
• Innfallet er morsomt = The idea is funny
  (adjective after er)

Even when the adjective comes after the verb, it still has to agree with the noun in gender and number.

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

So:

• jeg er = I am
• du er = you are
• innfallet er = the idea is

In this sentence, it simply links the subject innfallet with the description morsomt.

Because Norwegian usually puts the definite article at the end of the noun instead of using a separate word before it.

So English says:

• the idea

But Norwegian often says:

• innfallet

The the meaning is built into the ending -et.

Yes, very similar here.

The basic structure is:

• Innfallet = subject
• er = verb
• morsomt = subject complement / adjective

So it works like:

• The idea is funny

This is a very straightforward Norwegian sentence.

Yes, in many contexts you could use other words with a similar meaning, but the tone changes slightly.

• morsom / morsomt = funny, amusing
• artig = funny, amusing, entertaining
• gøy = fun, enjoyable

So:

• Innfallet er morsomt = the idea is funny/amusing
• Innfallet er artig = the idea is amusing
• Innfallet er gøy = the idea is fun

Morsomt often fits especially well if the idea itself is humorous.

A rough guide for an English speaker is:

INN-fal-et er MOR-shomt

A few helpful notes:

• innfallet has stress mainly on the first syllable: INN
• the ll in innfall is a normal Norwegian l
• morsomt is often pronounced with the final t clearly, though exact pronunciation can vary a bit by dialect

If you are learning Bokmål pronunciation, it is best to listen to native audio, since stress and melody are important in Norwegian.