Når det regner, setter vi opp tørkestativet i stuen.

This is because of the V2 rule in Norwegian.

In a main clause, the finite verb usually comes in second position. When the sentence starts with a time clause like Når det regner, that whole clause counts as the first element. So the verb in the main clause must come next:

• Når det regner, setter vi opp tørkestativet i stuen.

If you started directly with the subject, you would get:

• Vi setter opp tørkestativet i stuen når det regner.

Both are correct, but when the når-clause comes first, Norwegian switches the order to verb + subject in the main clause.

Here det is a dummy subject, just like it in English it is raining or it rains.

Weather expressions in Norwegian often use det:

• Det regner = it is raining / it rains
• Det snør = it is snowing
• Det blåser = it is windy / it is blowing

So det does not refer to a real thing. It is simply required because Norwegian finite clauses normally need a subject.

Når is commonly used for something that happens whenever or when in a general/repeated sense.

In this sentence, the meaning is something like:

• whenever it rains, we put up the drying rack in the living room

So når fits well because this is a habit or a general situation.

By contrast, da is often used for one specific time in the past:

• Da det regnet i går, satte vi opp tørkestativet i stuen.
  = When it rained yesterday, we put up the drying rack in the living room.

A useful shortcut:

• når = when / whenever, often general or repeated
• da = when, often one completed event in the past

Because sette opp is a phrasal/separable verb.

The base verb is sette, and opp adds an extra meaning, here something like put up, set up, or unfold/put into use depending on context.

In a main clause, the finite verb usually comes earlier, while the particle often stays later in the sentence:

• Vi setter opp tørkestativet.

But in the infinitive or some other forms, they are often kept together:

• å sette opp tørkestativet
• vi skal sette opp tørkestativet
• vi har satt opp tørkestativet

So this split is normal Norwegian word order.

In this sentence, sette opp tørkestativet means something like:

• put up the drying rack
• set up the drying rack
• unfold and place the drying rack so clothes can dry

It does not mean only put it upward in a literal physical sense. The combination sette opp works as a unit and has an idiomatic meaning.

Norwegian often uses verb + particle combinations this way, just like English uses set up, put on, take off, and so on.

Because tørkestativet is the definite form: the drying rack.

The noun is:

• et tørkestativ = a drying rack
• tørkestativet = the drying rack

Norwegian usually adds definiteness as an ending on the noun instead of using a separate word like the.

So:

• Vi setter opp et tørkestativ = We put up a drying rack
• Vi setter opp tørkestativet = We put up the drying rack

The sentence uses the definite form because it probably refers to a specific drying rack the speakers already know about.

I stuen means in the living room.

Here:

• i = in
• stuen = the living room

For rooms, Norwegian often uses i where English also uses in:

• i stuen = in the living room
• i kjøkkenet = in the kitchen
• i gangen = in the hallway

So this is a very natural choice of preposition.

Also, stuen is the definite form of stue:

• en stue = a living room / lounge
• stuen = the living room

In many dialects and informal Bokmål, you may also see stua instead of stuen.

Yes, in most cases they mean the same thing: in the living room.

This is mainly a difference in style or form:

• stuen is a common Bokmål definite form
• stua is also very common and often feels a bit more everyday or spoken

So both of these are natural:

• i stuen
• i stua

A learner should recognize both.

Yes, this comma is standard and natural.

The first part, Når det regner, is a subordinate clause, and it is followed by the main clause:

• Når det regner, setter vi opp tørkestativet i stuen.

In Norwegian, a subordinate clause placed before the main clause is normally separated by a comma.

So the comma helps show the structure clearly.

Yes, absolutely.

That version is also correct:

• Vi setter opp tørkestativet i stuen når det regner.

The difference is mostly about focus and sentence flow:

• Når det regner, setter vi opp tørkestativet i stuen.
  This emphasizes the condition/time first: when it rains...

• Vi setter opp tørkestativet i stuen når det regner.
  This starts with what we do, and adds the condition later.

Both are natural.

The present tense is used because the sentence describes a general habit or usual practice.

• regner = rains / is raining
• setter = put / set up

In English, we also often use the present for repeated situations:

• When it rains, we put up the drying rack in the living room.

So the present tense here does not mean it is happening only right now. It can mean this is what we normally do whenever it rains.

Yes, roughly.

Tørkestativ is a compound noun:

• tørke relates to drying
• stativ means stand / rack

So tørkestativ is literally something like drying rack or clothes drying rack.

This is very typical in Norwegian: nouns are often joined into one word.

Other examples:

• spisebord = dining table
• vaskemaskin = washing machine
• bokhylle = bookshelf

So if a long Norwegian noun looks intimidating, it often helps to split it into parts.

No, that would not be correct in standard Norwegian.

You need det because Norwegian clauses normally require a subject, and weather verbs use the dummy subject det:

• Når det regner = correct
• Når regner = incorrect

This is very similar to English, where you say when it rains, not when rains.