Det var en vanvittig plan.

In this sentence, det can often be understood as it or that, depending on context.

• Det var en vanvittig plan. = It was an insane plan.
• In natural English, you might also translate it as That was an insane plan if you are referring back to something already mentioned.

Norwegian uses det very often in places where English uses it, that, or even a more general subject.

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

• er = is / am / are
• var = was / were

So:

• Det er en vanvittig plan. = It is an insane plan.
• Det var en vanvittig plan. = It was an insane plan.

Because plan is a common-gender noun in Norwegian, and in the singular indefinite it takes en.

• en plan = a plan
• planen = the plan

So the structure is:

• en vanvittig plan = an insane plan

This is similar to English a/an, but in Norwegian the article also shows the grammatical gender of the noun.

Here, vanvittig is the correct form because it is describing a singular indefinite common-gender noun:

• en vanvittig plan

Very roughly, adjective endings often work like this:

• en vanvittig plan
• ei vanvittig bok
• et vanvittig prosjekt
• vanvittige planer
• den vanvittige planen

So in this sentence, vanvittig is exactly the form you expect.

Because plan here is indefinite: a plan, not the plan.

Compare:

• en plan = a plan
• planen = the plan

So:

• Det var en vanvittig plan. = It was an insane plan.
• Det var den vanvittige planen. = That/It was the insane plan.

In Norwegian, definiteness is often marked on the noun itself with an ending like -en, -a, or -et.

Vanvittig is an adjective. It means something like:

• insane
• crazy
• mad
• sometimes ridiculous or extreme, depending on context

In en vanvittig plan, it describes the noun plan.

It can also sometimes be used informally as an adverb-like intensifier in spoken language, but here it is clearly an adjective.

Yes, it is a fairly strong and expressive word, but it is completely natural.

Vanvittig can suggest that something is:

• wildly unrealistic
• shockingly bold
• absurd
• recklessly extreme

So Det var en vanvittig plan could mean:

• the plan was absurd
• the plan was reckless
• the plan was unbelievably daring

The exact nuance depends on context and tone.

Yes, but the tone changes a bit.

Some rough comparisons:

• vanvittig = insane / mad / outrageous
  Fairly strong, expressive, common.
• gal = crazy / mad
  Often a bit simpler and more everyday.
• sinnssyk = insanely crazy / psychotic / totally insane
  Usually stronger and more intense, and can sound more informal or dramatic.

So all of these are possible, but vanvittig has a natural dramatic feel that fits well here.

The word order is very standard Norwegian:

• Det = subject
• var = verb
• en vanvittig plan = subject complement

So the pattern is:

Subject + verb + complement

That is why it looks very similar to English:

• It was an insane plan.

A rough pronunciation guide for an English speaker could be:

• Det ≈ deh
• var ≈ vahr
• en ≈ en or weak ən in fast speech
• vanvittig ≈ VAHN-vit-tih
• plan ≈ plahn

A few important notes:

• Norwegian r varies by region.
• The a in var and plan is usually broader than in English cat.
• In natural speech, det and en may be reduced somewhat.

You do not need perfect pronunciation to be understood, but keeping the stress on the first syllable of vanvittig helps.

Normally, no. Norwegian usually requires an explicit subject in a sentence like this.

So you say:

• Det var en vanvittig plan.

Not:

• Var en vanvittig plan.

English also usually needs a subject here, so the structure matches fairly well.

Yes, the meaning is close, but the focus is slightly different.

• Det var en vanvittig plan.
  This presents or identifies it as an insane plan.
• Planen var vanvittig.
  This puts more focus on the plan itself and says it was insane.

Also, planen means the plan, so it is definite, not indefinite.

Usually it is figurative.

Most of the time, en vanvittig plan means a plan that is:

• absurd
• reckless
• wildly unrealistic
• unbelievably bold

It does not usually mean that someone was literally medically insane. It is much more often just strong everyday language. Context decides how harsh or playful it sounds.