Hun sætter sig i sofaen og læser avisen.

Because at sætte sig means to sit down or to seat oneself. It is a reflexive expression, so it uses sig.

• sætte = to put / place / set
• sætte sig = to sit down

So in this sentence, Hun sætter sig i sofaen means she is moving into a seated position, not just already sitting there.

Sig is the reflexive pronoun in Danish. Here it means something like herself, but Danish often uses the reflexive pronoun where English would not say it explicitly.

So:

• Hun sætter sig = She sits down
• more literally: She sets herself

You use sig when the subject and the object refer to the same person.

Because sig is used when the action goes back to the subject of the clause.

• Hun sætter sig = she sits herself down / she sits down
• Hun sætter hende would mean she is placing another female person

So sig refers back to hun, while hende would refer to someone else.

Yes, literally i sofaen is in the sofa, but in Danish this is the normal idiomatic way to say on the sofa or in the sofa, depending on the context.

Danish often uses i where English uses on for furniture or places you sit inside or down into.

So:

• i sofaen = on the sofa / in the sofa

This is just a normal Danish way of expressing it.

Because sofaen is the definite form: the sofa.

In Danish, definiteness is often shown by adding an ending to the noun:

• en sofa = a sofa
• sofaen = the sofa

The same happens with:

• en avis = a newspaper
• avisen = the newspaper

So the sentence uses definite nouns: the sofa and the newspaper.

In Danish, many nouns make the definite form by adding a suffix.

For common-gender nouns:

• en sofa → sofaen
• en avis → avisen

So instead of putting a separate word before the noun, Danish often attaches the definite article to the end.

This is one of the biggest structural differences from English.

Because Danish, like English, often leaves out a repeated subject in coordinated clauses when it is understood.

So:

• Hun sætter sig i sofaen og læser avisen

means:

• She sits down on the sofa and reads the newspaper

You could think of the second part as having an understood hun:

• Hun sætter sig i sofaen og (hun) læser avisen

Leaving it out sounds natural.

Yes. Læser is the present tense of at læse.

• at læse = to read
• læser = reads / is reading

In Danish, the present tense can often translate either as simple present or present continuous in English, depending on context.

So hun læser avisen can mean:

• she reads the newspaper
• she is reading the newspaper

Here, the second meaning is usually the natural one.

This is a very common question.

• sætter sig = sits down
  This describes the movement/change into a sitting position.
• sidder = is sitting / sits
  This describes the state/position of already being seated.

So:

• Hun sætter sig i sofaen = She sits down on the sofa.
• Hun sidder i sofaen = She is sitting on the sofa.

The sentence uses sætter sig because it describes the action of sitting down first.

This is normal Danish main-clause word order:

• Hun = subject
• sætter = finite verb
• sig = reflexive pronoun
• i sofaen = prepositional phrase
• og læser avisen = coordinated second action

Danish main clauses usually place the finite verb in second position. In this sentence, the subject comes first, so the verb comes right after it:

• Hun sætter ...

This is one of the basic word-order patterns in Danish.

In many contexts, yes, and native speakers may use both depending on nuance and dialectal preference. But i sofaen is very natural in Danish for sitting down in/on a sofa.

Very roughly:

• i sofaen can suggest being seated in it, almost as an enclosed sitting space
• på sofaen can sound more like on the sofa

In practice, i sofaen is extremely common and natural, so it is a good phrase to learn as a set expression.

Normally it suggests a natural sequence:

1. She sits down on the sofa.
2. She reads the newspaper.

But in real usage, the sentence presents the two actions as part of one scene, so the second action follows naturally from the first. Danish does not need any extra word here to make that sequence clear.

So the sentence gives the picture: she sits down and then reads.