Hun lægger sig på sofaen, når hun er træt.

Because Danish separates the action of moving into a position from the state of already being in that position.

• lægge sig = to lie down, to put oneself into a lying position
• ligge = to be lying

So:

• Hun lægger sig på sofaen = she lies down on the sofa
• Hun ligger på sofaen = she is lying on the sofa

This is a very common distinction in Danish, and it does not match English exactly.

Sig is a reflexive pronoun. Here it means herself.

So lægger sig is literally something like lays herself down, but in natural English we usually just say lies down.

You often need sig in Danish with verbs where English does not use herself/himself explicitly.

Compare:

• Han sætter sig = he sits down
• Hun lægger sig = she lies down

Because sig is the normal reflexive form when the subject and object refer to the same person.

• hun lægger sig = she lies down / she lays herself down
• hun lægger hende would mean she lays her, referring to some other female person

So sig shows that the action comes back to the subject.

Because sofaen is the definite form, meaning the sofa.

Danish usually adds the definite article to the end of the noun:

• en sofa = a sofa
• sofaen = the sofa

This is one of the basic noun patterns in Danish.

Because Danish prepositions do not always match English one-for-one, and på is natural here with lægger sig.

The phrase lægger sig på sofaen focuses on moving onto the sofa and ending up there.

In furniture expressions, Danish may use different prepositions depending on the verb and the image being used. For example, you may also hear things like sidde i sofaen. So it is best to learn the whole expression, not just translate each word mechanically.

Here når means when or whenever. It is used for something that happens regularly, typically, or whenever a certain situation occurs.

That fits this sentence because it describes a habit or usual pattern.

Compare:

• når = when/whenever
• hvis = if
• da = when, about a specific past event

So this sentence is not about a one-time past event, and it is not really presenting the tiredness as just a hypothetical condition. It is saying: whenever she is tired, this is what she does.

Because Danish, like English, often uses the present tense for habits and general truths.

• lægger = present tense
• er = present tense

So the sentence is not necessarily about what she is doing right now. It can mean this is her usual behavior.

Because når hun er træt is a subordinate clause, and Danish normally places a comma before subordinate clauses.

So:

• Hun lægger sig på sofaen, når hun er træt.

This is standard Danish punctuation.

Because subordinate clauses in Danish do not use normal main-clause word order.

After a conjunction like når, the order is usually:

• conjunction + subject + verb

So:

• når hun er træt = when she is tired

not

• når er hun træt

A useful extra point: if you move the når-clause to the front, the main clause changes word order:

• Når hun er træt, lægger hun sig på sofaen.

That inversion in the main clause is very typical Danish.

Because the subject is singular: hun.

Predicative adjectives in Danish agree in number:

• hun er træt = she is tired
• de er trætte = they are tired

So træt is the correct singular form here.