Hun har en blå nederdel på i dag, fordi vejret er varmt nok.

Because have på is a common Danish verb phrase meaning to have on / to be wearing.

In a main clause, the object usually goes between the verb and på:

• Hun har en blå nederdel på. = She is wearing a blue skirt.
• Han har en hat på. = He is wearing a hat.

So på belongs with har, but it appears later in the sentence because Danish often splits this kind of expression.

No, that sounds wrong in normal Danish.

With have ... på for clothing, the usual order is:

• subject + har + clothing item + på

So:

• Hun har en blå nederdel på. ✓
• Hun har på en blå nederdel. ✗

Yes. Very literally, it means She has a blue skirt on, and in natural English that usually becomes She is wearing a blue skirt.

Danish often uses have ... på where English uses wear.

Because nederdel is a singular common-gender noun.

That gives you:

• en nederdel = a skirt
• en blå nederdel = a blue skirt

The adjective blå does not change here because it is:

• singular
• indefinite
• common gender

Compare:

• en blå nederdel = a blue skirt
• et blåt hus = a blue house

So the form depends on the gender and form of the noun.

In Danish, adjectives often change form, but blå stays the same in this sentence because it modifies a singular common-gender noun in the indefinite form.

Basic pattern:

• en blå nederdel = common gender singular indefinite
• et blåt hus = neuter singular indefinite
• de blå nederdele = plural definite/indefinite

So blå is the correct form here.

I dag is the standard fixed expression meaning today.

Examples:

• Jeg arbejder i dag. = I am working today.
• Hun har en blå nederdel på i dag. = She is wearing a blue skirt today.

Danish normally uses i dag, not bare dag, for this meaning.

It can appear in different places, depending on emphasis.

In your sentence:

• Hun har en blå nederdel på i dag.

You could also say:

• I dag har hun en blå nederdel på.

Both are correct. The version with I dag first gives a little more emphasis to today.

Because Danish very often uses the definite form when talking about the weather in a general or current sense.

So:

• vejret = the weather

In this sentence:

• fordi vejret er varmt nok = because the weather is warm enough

This is very natural Danish. Using just vejr here would sound wrong.

Because vejr is a neuter noun:

• et vejr

When an adjective is used predicatively after er, it still agrees with a neuter singular noun:

• Vejret er varmt. = The weather is warm.

Compare:

• Bilen er varm. = The car is warm. (bil is common gender)
• Vejret er varmt. = The weather is warm. (vejr is neuter)

That is why you need varmt.

Here nok means enough or sufficiently.

So:

• varmt nok = warm enough

Examples:

• Det er varmt nok til at sidde udenfor. = It is warm enough to sit outside.
• Vejret er varmt nok. = The weather is warm enough.

So nok does not mean probably here. It means enough.

Because fordi introduces a subordinate clause, and Danish uses different word order in subordinate clauses.

Main clause word order often has the finite verb in second position:

• Vejret er varmt.

But after fordi, the clause becomes subordinate, so the subject normally comes before the finite verb:

• fordi vejret er varmt nok

That is standard Danish subordinate-clause order.

No. That would be wrong in standard Danish.

After fordi, the correct order is:

• fordi vejret er varmt nok

not:

• fordi er vejret varmt nok ✗

So remember: after fordi, use normal subordinate word order with subject + verb.

Fordi means because. It introduces the reason.

So the sentence structure is:

• statement: Hun har en blå nederdel på i dag
• reason: fordi vejret er varmt nok

Together:

• She is wearing a blue skirt today because the weather is warm enough.

It is very commonly used for clothes, shoes, glasses, hats, and accessories—basically things you have on your body.

Examples:

• Hun har en jakke på. = She is wearing a jacket.
• Han har briller på. = He is wearing glasses.
• Jeg har sko på. = I am wearing shoes.

So it is broader than just clothing, but it is mainly used for things worn on the body.

Yes, but have ... på is one of the most common and natural ways to say wear in everyday Danish.

You may also see:

• Hun bærer en blå nederdel.

That literally uses a verb like wear/carry, but it can sound more formal, literary, or stylistically marked depending on context.

For everyday speech, Hun har en blå nederdel på is very natural.

Because Danish punctuation normally uses a comma before a subordinate clause such as one introduced by fordi.

So:

• Hun har en blå nederdel på i dag, fordi vejret er varmt nok.

That comma is standard in Danish writing.

Yes, that is the natural interpretation.

The sentence suggests a connection between the warm weather and her clothing choice:

• She is wearing a blue skirt today because the weather is warm enough.

So the weather is presented as the reason she is wearing that skirt today.

It is just an ordinary adjective + noun combination.

• blå = blue
• nederdel = skirt

So you can easily swap the adjective:

• en sort nederdel = a black skirt
• en kort nederdel = a short skirt
• en flot nederdel = a nice skirt

Nothing idiomatic is happening there.

Nederdel means skirt.

It is a common-gender noun:

• en nederdel = a skirt
• nederdelen = the skirt
• nederdele = skirts
• nederdelene = the skirts

So in your sentence:

• en blå nederdel = a blue skirt