Planen virker idiotisk.

Because planen is the definite singular form of en plan.

• en plan = a plan
• planen = the plan

In Norwegian, definiteness is often marked by adding an ending to the noun instead of using a separate word like the.

So:

• En plan virker idiotisk. = A plan seems idiotic.
• Planen virker idiotisk. = The plan seems idiotic.

The dictionary form is plan.

You would normally learn it as:

• en plan = a plan

From that, you get:

• planen = the plan
• planer = plans
• planene = the plans

Here, virker means seems, appears, or comes across as.

It is the present tense of å virke.

So:

• å virke = to seem / to appear / to work, depending on context
• virker = seems / appears / works

In this sentence, because it is followed by an adjective (idiotisk), it means seems:

• Planen virker idiotisk. = The plan seems idiotic.

Yes. That is a very common source of confusion.

Å virke can mean both:

1. to work / function
2. to seem / appear

Examples:

• Maskinen virker. = The machine works.
• Planen virker idiotisk. = The plan seems idiotic.

The context tells you which meaning is intended.

Because the verb here is virker, not er.

Compare:

• Planen er idiotisk. = The plan is idiotic.
• Planen virker idiotisk. = The plan seems idiotic.

So virker already carries the idea of seems. You do not add er as well.

It is an adjective.

It describes planen, but it comes after the verb virker. That makes it a predicative adjective.

Compare:

• en idiotisk plan = an idiotic plan
  
  • adjective before the noun
• Planen virker idiotisk. = The plan seems idiotic.
  
  • adjective after the verb

Because adjectives ending in -isk usually do not add -t in the neuter singular.

So you get:

• en idiotisk plan
• et idiotisk forslag

Not idiotiskt.

This is normal for many adjectives ending in -isk, -sk, and often -lig.

You do, however, get -e in the plural and in many definite forms:

• idiotiske planer = idiotic plans
• Planene virker idiotiske. = The plans seem idiotic.

Yes, it can sound fairly strong or blunt.

It usually means something like:

• idiotic
• stupid
• ridiculous in a harsh sense

If you want something softer, Norwegian might use words like:

• dum = stupid
• dårlig = bad
• lite gjennomtenkt = not well thought out
• uheldig = unfortunate
• merkelig = strange

So Planen virker idiotisk is not especially polite or neutral.

Because this is a normal Norwegian main clause, and Norwegian usually follows the verb-second pattern.

In a plain statement, the finite verb comes early:

• Planen virker idiotisk.

If you turn it into a yes/no question, the verb comes first:

• Virker planen idiotisk? = Does the plan seem idiotic?

That word order difference is very important in Norwegian.

Yes, but it is not exactly the same.

• Planen virker idiotisk = The plan seems idiotic
• Planen ser idiotisk ut = The plan looks idiotic

Ser ... ut is more about how something looks, while virker is broader and can mean the overall impression something gives.

For a plan, virker idiotisk is often more natural unless you are specifically talking about how it looks on paper or in presentation.

It is present tense.

The verb is:

• å virke = infinitive
• virker = present tense

So the sentence is talking about how the plan seems now.

A past version could be:

• Planen virket idiotisk. = The plan seemed idiotic.

Then you would be making the adjective part of the noun phrase itself.

Compare:

• Den idiotiske planen = the idiotic plan
• Planen virker idiotisk = the plan seems idiotic

The first one directly labels the plan as idiotic.
The second one presents that as an impression or judgment.

So they are related, but not identical in tone.

A rough guide is:

PLAH-nen VEER-ker ee-dee-OH-tisk

A few notes:

• planen has two syllables: pla-nen
• virker also has two syllables: vir-ker
• idiotisk is stressed around OH

A more Norwegian-like pronunciation will depend on dialect, but this rough version is enough to get started.