Hun siger, at hendes ryg gør ondt, og at smerten kommer igen, når hun sidder længe.

Because there are two subordinate clauses, both depending on Hun siger:

• at hendes ryg gør ondt
• at smerten kommer igen

In English, you can often say She says that her back hurts and that the pain comes back, and the second that is optional in many cases. In Danish, repeating at is very common and often clearer.

You may also hear people omit the second at in speech, but here repeating it is perfectly natural and good standard Danish.

This is a very common learner question.

Danish uses sin/sit/sine for a reflexive possessor, meaning the possession belongs to the subject of the same clause.

Here the relevant clause is:

• at hendes ryg gør ondt

The subject of this clause is hendes ryg, not hun. So hun is not the subject of the clause where the possession appears. Because of that, Danish uses hendes, not sin.

Compare:

• Hun vasker sin bil. = She washes her own car.
  
  • subject = hun
  • owner of the car = hun
  • same clause, so sin

But here:

• Hun siger, at hendes ryg gør ondt.
  • in the subordinate clause, subject = hendes ryg
  • owner = hun
  • not the same subject, so hendes

Because Danish often uses a possessive + noun without the definite ending.

So:

• hendes ryg = her back
• not hendes ryggen

This is similar to English, where we say her back, not her the back.

In Danish, the definite ending and a possessive usually do not go together:

• min bil = my car
• deres hus = their house
• hendes ryg = her back

Gør ondt literally means something like does hurt or causes pain, but in natural English it usually means hurts.

So:

• Min arm gør ondt. = My arm hurts.
• Det gør ondt. = It hurts.

This is a fixed expression in Danish. You do not normally say ryggen smerter in everyday speech, even though smerte exists as a noun and verb-related form. Gør ondt is the normal spoken way.

Igen means again.

So:

• smerten kommer igen = the pain comes back / returns

Danish often uses komme igen for something recurring:

• Feberen kommer igen. = The fever returns.
• Problemet kommer igen. = The problem comes back.

It sounds natural here because the sentence is describing repeated pain, especially under certain conditions.

Because the sentence refers to the pain, not just pain in general.

• smerte = pain
• smerten = the pain

The definite ending -en is attached to the noun:

• en smerte = a pain
• smerten = the pain

Here it refers to a specific pain already understood from the first part of the sentence: the pain in her back.

Because når is used for something that happens when/whenever a situation occurs, especially something real, repeated, or expected.

• når hun sidder længe = when/whenever she sits for a long time

Hvis means if and is more conditional or uncertain:

• Hvis hun sidder længe, får hun ondt. = If she sits for a long time, she gets pain.

In many contexts, both may seem possible, but når fits best when describing a recurring pattern that actually happens.

Both can work, but sidder længe is very common and idiomatic.

• længe = for a long time
• sidder længe = sits for a long time / has been sitting a long time

You can also say:

• sidder i lang tid

But sidder længe is shorter and very natural.

A useful distinction:

• længe = duration over a long period
• lang tid = a long time, as a noun phrase

So Danish often prefers the adverb længe in this kind of sentence.

Because after når, Danish uses subordinate clause word order, which is generally:

• conjunction + subject + verb + other elements

So:

• når hun sidder længe

This is normal subordinate-clause structure.

If a sentence begins with the subordinate clause, the main clause after it will usually have verb-second word order:

• Når hun sidder længe, kommer smerten igen.

Here kommer comes before smerten because the whole Når ... clause takes first position.

No. Danish needs the subject to be stated here.

So you say:

• når hun sidder længe

not:

• når sidder længe

Unlike some languages, Danish normally does not drop subject pronouns like this. Each finite clause needs its own subject.

Because Danish comma rules usually mark off subordinate clauses.

In this sentence, you have:

• Hun siger
• at hendes ryg gør ondt
• og at smerten kommer igen
• når hun sidder længe

The comma before the first at marks the start of a subordinate clause.
The comma before og helps separate coordinated clause parts.
The comma before når marks another subordinate clause.

Comma usage in Danish can vary a little depending on whether someone follows traditional comma or new comma rules, but the version you were given is standard and natural.

Yes, it often can:

• Hun siger, at hendes ryg gør ondt, og smerten kommer igen, når hun sidder længe.

This is possible, especially in speech or less formal writing. But repeating at makes the structure clearer, because both parts are explicitly linked to Hun siger.

So:

• ... og at smerten kommer igen ... = very clear and elegant
• ... og smerten kommer igen ... = also possible in many contexts

No. It is very common for body parts, but it can also be used more generally.

Examples:

• Min nakke gør ondt. = My neck hurts.
• Det gør ondt i ryggen. = It hurts in the back.
• Det gør ondt at høre. = It hurts to hear that.
• Det gjorde ondt på ham. = It hurt him.

So it can describe physical pain and sometimes emotional pain too.

Yes, but the structure would be different.

You can say:

• Hun siger, at hun har ondt i ryggen. = She says she has pain in her back.
• Hun siger, at hendes ryg gør ondt. = She says her back hurts.

Both are natural. The version with har ondt i ryggen is probably even more common in everyday speech. The given sentence is still completely correct and natural.

It is in the present tense throughout:

• siger = says
• gør = hurts / does hurt
• kommer = comes
• sidder = sits

Danish present tense is often used for:

• current situations
• general truths
• repeated events

So here it describes an ongoing or recurring health problem.