Det er dumt.

Because det is a neuter subject, and adjectives used predicatively after er usually agree with the subject.

• en dum idé = a stupid idea
• et dumt spørgsmål = a stupid question
• Det er dumt = It is stupid / That is stupid

The basic adjective is dum.
The -t form is used with et-words and with det in sentences like this.

Det usually means it or that, depending on context.

In Det er dumt, it often works like English it or that:

• Det er dumt. = It’s stupid. / That’s stupid.

Danish uses det very often in general statements, even where English might choose either it or that depending on the situation.

Danish is a V2 language, which means the finite verb normally comes in the second position in main clauses.

So in:

• Det = first element
• er = verb in second position
• dumt = complement

This word order is very standard in Danish main sentences.

A careful pronunciation is roughly:

• det ≈ deh or sometimes very reduced
• er ≈ air / a very weak er
• dumt ≈ doomt

A few useful notes:

• t in det is often weak or barely heard in normal speech.
• er is often unstressed and reduced.
• dumt ends in a consonant cluster, so the final -mt may feel a little compact for English speakers.

If you say it slowly and clearly as det er dumt, you will still be understood.

Not always. Dum/dumt can mean things like:

• stupid
• foolish
• silly
• a bad idea

So Det er dumt could mean:

• That’s stupid
• That’s silly
• That’s a foolish thing to do

The best English translation depends on the context and tone.

Yes, but be careful with tone.

Examples:

• Det er dumt. = That is stupid.
• Han er dum. = He is stupid.
• Hun er dum. = She is stupid / mean, depending on context.

When used about a person, dum can sound quite direct and harsh.
When used about an action, idea, or situation, it often sounds more natural and less personal.

Because dumt is an adjective complement, not a noun phrase.

In Det er dumt, the structure is:

• subject: det
• verb: er
• adjective: dumt

You only use en/et if a noun follows:

• Det er en dum idé. = It is a stupid idea.
• Det er dumt. = It is stupid.

So en is not possible unless you also add a noun.

You normally put ikke after the verb:

• Det er ikke dumt. = It is not stupid.

This is the usual position of ikke in a simple main clause.

Yes, but they mean slightly different things.

• Det er dumt = It/that is stupid. Very common and general.
• Dette er dumt = This is stupid. More explicit, a bit more formal or pointed.
• Den er dum = It is stupid, but den refers to a common-gender noun already mentioned.
• Det er dumt can also refer more generally to a situation, idea, or unnamed thing.

For example:

• Bogen? Den er dum. = The book? It’s stupid.
• At gøre det nu? Det er dumt. = Doing it now? That’s stupid.

You change the verb to the past tense:

• Det var dumt. = That was stupid.

The adjective stays dumt because it still agrees with det. Only the verb changes:

• er = is
• var = was