Feilen var idiotisk.

Because feilen is the definite singular form: the mistake / the error.

The noun works like this:

• feil = error, mistake
• en feil = a mistake
• feilen = the mistake

So the sentence is talking about a specific mistake, not just mistakes in general.

It is singular.

This is a good word to watch out for, because the forms can look confusing:

• en feil = a mistake
• feilen = the mistake
• feil = mistakes / errors (plural indefinite)
• feilene = the mistakes / the errors

So feil can be either the basic dictionary form or the plural indefinite form, depending on context.

Var is the past tense of å være (to be).

Useful forms:

• å være = to be
• er = is / am / are
• var = was / were
• har vært = has/have been

So Feilen var idiotisk is in the past: the mistake was idiotic.

Here idiotisk is a predicative adjective, meaning it comes after a linking verb like er or var.

In Norwegian, predicative adjectives agree with the noun in gender/number, but:

1. For a singular common-gender noun, the basic form is used.
2. Adjectives ending in -isk usually do not add -t in the neuter singular either.

So:

• Feilen var idiotisk = common gender singular
• Problemet var idiotisk = neuter singular
• Feilene var idiotiske = plural

That last one shows the change more clearly: in the plural, you get idiotiske.

Yes.

That is the adjective used before the noun:

• en idiotisk feil = an idiotic mistake

Compare:

• Feilen var idiotisk. = The mistake was idiotic.
• Det var en idiotisk feil. = It was an idiotic mistake.

So both are correct, but they are structured differently.

No, this is a very basic Norwegian sentence pattern:

• Feilen = subject
• var = verb
• idiotisk = predicative adjective

So it follows the simple pattern:

Subject + verb + complement

That said, Norwegian main clauses usually follow V2 word order, meaning the finite verb comes in the second position. In this sentence, the subject is first, so the verb naturally comes second.

Yes, it is fairly strong.

It means something like idiotic, absurdly stupid, or ridiculously foolish. It sounds harsher than softer words like:

• dum = stupid, dumb
• teit = silly, dumb, lame

When used about a mistake, decision, or action, it criticizes it strongly. It does not automatically mean you are calling a person an idiot; you are describing the mistake itself.

It can sound blunt, but it is natural Norwegian.

Whether it sounds rude depends on context. Saying a mistake was idiotisk is strong criticism. In conversation, Norwegians do use strong adjectives, but tone and situation matter.

If you wanted something milder, you could say:

• Feilen var dum. = The mistake was stupid.
• Feilen var veldig uheldig. = The mistake was very unfortunate.

A rough English-style guide would be:

• Feilen ≈ FYE-len
• var ≈ vahr
• idiotisk ≈ ee-dee-O-tisk

The main stress in idiotisk is on the o syllable: idio-TISK is not right; think more like ee-dee-O-tisk.

Exact pronunciation varies somewhat by dialect, but that rough guide will get you close.

Not in standard Bokmål if you mean the mistake in the singular.

In standard Bokmål:

• feilen = the mistake

You may hear feila in some dialects or very informal speech, but for standard written Bokmål, feilen is the form to use here.

Be careful also not to confuse it with plural-related forms in speech and dialects. For a learner, feilen is the safe standard choice.