Det var en dum idé.

• Det = it / that
• var = was
• en = a / an
• dum = stupid, foolish, silly
• idé = idea

So the structure matches English quite closely.

Because idé is an en-word in Swedish, not an ett-word.

In Swedish, nouns have grammatical gender:

• en words
• ett words

Since idé is an en noun, you say:

• en idé
• en dum idé

The adjective must match the noun type:

• with an en-word: dum
• with an ett-word: dumt

So:

• en dum idé = correct
• ett dumt idé = incorrect

Swedish adjectives change depending on the noun they describe.

With indefinite singular nouns:

• en-word → basic form: dum
• ett-word → dumt
• plural → dumma

Examples:

• en dum idé = a stupid idea
• ett dumt förslag = a stupid suggestion
• dumma idéer = stupid ideas

Because idé is an en-word and singular, dum stays in its basic form.

Var is the past tense of vara (to be).

Common forms:

• är = is / am / are
• var = was / were
• har varit = has been / have been

So:

• Det är en dum idé. = It is a stupid idea.
• Det var en dum idé. = It was a stupid idea.

Det often means it or that, and Swedish uses it very naturally in sentences like this when referring to a situation, action, or idea already understood from context.

So Det var en dum idé is the normal way to say:

• That was a stupid idea
• It was a stupid idea

You are not necessarily talking about a physical object called it. Det can refer more generally to something that happened or was suggested.

Yes, but it is a little different.

• Det var en dum idé = That/it was a stupid idea.
• Idén var dum = The idea was stupid.

The second version names the idea directly and uses the definite form idén (the idea).

So both are correct, but they do not have exactly the same focus:

• Det var en dum idé comments on a situation or suggestion.
• Idén var dum talks directly about the idea itself.

Yes. Dum can mean:

• stupid
• foolish
• silly
• sometimes bad in a mild sense, depending on context

So en dum idé could be:

• a stupid idea
• a foolish idea
• a silly idea

In many situations, foolish or silly may sound more natural in English than the stronger word stupid.

The accent in idé is part of the spelling. It helps show the stress and pronunciation.

The word is pronounced with stress on the last syllable, roughly like:

• ee-DAY

This is different from many native Swedish words, which are often stressed earlier.

So the accent is not optional here; it belongs to the word.

You just change var to är:

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

So:

• är = present
• var = past

Yes. Det var dumt means That was stupid or It was stupid.

The difference is:

• Det var en dum idé specifically says it was a stupid idea
• Det var dumt is more general and does not name the idea

So if you want to mention the idea itself, use Det var en dum idé. If the context is already clear, Det var dumt is very common.

The plural would be:

• Det var dumma idéer. = They were stupid ideas. / Those were stupid ideas.

Changes:

• idé → idéer
• dum → dumma

Notice that in Swedish, plural indefinite nouns usually do not take en or ett, so you just say:

• dumma idéer = stupid ideas