Jeg strækker ryggen, når jeg har siddet længe ved skrivebordet.

In Danish, body parts are often expressed with the definite form rather than a possessive, when it is already obvious whose body part is meant.

So:

• Jeg strækker ryggen = literally I stretch the back
• but it naturally means I stretch my back

This is very common in Danish. You will also see things like:

• Jeg vasker hænderne = I wash my hands
• Hun løfter armen = She raises her arm

Using min ryg is possible, but it often sounds more emphatic or contrastive, as if you really want to stress my back.

Strækker is the present tense of at strække.

Danish uses the present tense here for a habitual action, just like English does:

• Jeg strækker ryggen = I stretch my back
• meaning: this is something I generally do in that situation

So the sentence describes a regular habit, not something happening only right now.

Here når means when in the sense of whenever or every time that.

So:

• Jeg strækker ryggen, når jeg har siddet længe ved skrivebordet
• = I stretch my back when I’ve been sitting at the desk for a long time

A useful distinction:

• når = when / whenever for repeated or general situations
• da = when for a specific event in the past
• hvis = if

Examples:

• Når jeg er træt, går jeg tidligt i seng. = Whenever I’m tired, I go to bed early.
• Da jeg var barn, boede jeg i Aarhus. = When I was a child, I lived in Aarhus.
• Hvis jeg er træt, drikker jeg kaffe. = If I’m tired, I drink coffee.

Har siddet is the present perfect of sidde.

• jeg har siddet = I have sat / I have been sitting

In this sentence, Danish uses the present perfect because the sitting happens before the stretching, and it is relevant to the present situation. The idea is:

• after I have been sitting for a long time, I stretch my back

If you said jeg sad, that would be simple past and would sound more like a completed past story, not a general pattern tied to the present moment.

Yes.

The verb is:

• infinitive: at sidde = to sit
• present: sidder
• past: sad
• past participle: siddet

The perfect is formed with har + past participle:

• jeg har siddet

That is completely normal Danish grammar.

Because når jeg har siddet længe ved skrivebordet is a subordinate clause, and Danish word order in subordinate clauses is different from main clauses.

Here the order is:

• når
  • subject + auxiliary + participle + adverbial

So:

• når jeg har siddet længe ved skrivebordet

That is standard subordinate-clause order.

Compare:

• Jeg har siddet længe ved skrivebordet.
  main clause

• ...når jeg har siddet længe ved skrivebordet.
  subordinate clause

Yes. If the subordinate clause comes first, the main clause afterwards has inversion.

So you can say:

• Jeg strækker ryggen, når jeg har siddet længe ved skrivebordet.

But also:

• Når jeg har siddet længe ved skrivebordet, strækker jeg ryggen.

Notice the difference:

• not jeg strækker
• but strækker jeg

This is a very important Danish word-order rule: when something other than the subject comes first in the main clause, the verb usually comes before the subject.

Ved usually means by, at, or near depending on context.

Here:

• ved skrivebordet = at the desk

This is the natural Danish way to say someone is sitting and working/staying at a desk.

It does not necessarily mean physically pressed against the desk. It just means the person is in that desk-setting.

Skrivebordet is the definite form: the desk.

In this sentence, Danish naturally uses the definite form because it refers to the desk as a known, familiar setting — the desk the speaker means in context.

• ved skrivebordet = at the desk

If you said ved et skrivebord, it would mean at a desk, which is more indefinite and less natural in this kind of everyday statement.

Længe means for a long time or long as an adverb.

So:

• jeg har siddet længe = I have been sitting for a long time

Yes, i lang tid is also possible:

• jeg har siddet i lang tid

Both are correct.
A rough difference:

• længe is short and very common
• i lang tid is a bit more explicit, like for a long time

In this sentence, længe sounds very natural.

A rough guide for an English speaker:

• strækker: the æ is a front vowel, somewhat like the vowel in cat, but Danish pronunciation is not exactly the same as English
• ryggen: the y is a rounded front vowel, which English does not really have; you say something like ee but with rounded lips

A very rough approximation might be:

• strækker ≈ STREK-er
• ryggen ≈ something like RÜG-en

But these are only rough hints. The Danish vowels are important here, especially:

• æ
• y

So it is best to listen to native audio if possible.

You could in some contexts, but stretch my back is the closest basic translation.

• strække ryggen = stretch my back

Adding out in English may sound natural in some sentences, but it is not something you need to reflect directly from the Danish.

So for learning purposes, it is best to remember:

• strække = stretch
• ryggen = the back / my back in context

It describes a general habit.

That comes from two things:

• the present tense strækker
• the conjunction når, which often means whenever

So the sense is:

• Whenever I have been sitting at the desk for a long time, I stretch my back.

If you wanted a one-time past event, the Danish would usually be phrased differently.