Sofie bergt haar pyjama op, terwijl Anna al onder de deken ligt te slapen.

Because the full verb is opbergen, a separable verb.

• opbergen = to put away / tidy away / store
• In a main clause, Dutch often splits separable verbs:
  • Sofie bergt haar pyjama op.
• In forms like the infinitive or past participle, it stays together:
  • Ze wil haar pyjama opbergen.
  • Ze heeft haar pyjama opgeborgen.

So bergt is the conjugated part, and op moves to the end of the clause.

Here, opbergen means something like:

• to put away
• to tidy away
• to store

So Sofie bergt haar pyjama op suggests Sofie is putting her pajamas away, probably because she is done with them.

It is a very natural everyday Dutch verb for putting objects back where they belong.

haar here is the possessive adjective meaning her.

It refers to Sofie, so:

• Sofie bergt haar pyjama op = Sofie puts away her pajamas.

A useful point for English speakers: Dutch haar can mean:

• her as a possessive: haar jas = her coat
• her as an object pronoun in some contexts: Ik zie haar = I see her

Here it is clearly possessive because it comes before a noun: haar pyjama.

Because terwijl introduces another clause.

The sentence has two parts:

• Sofie bergt haar pyjama op
• terwijl Anna al onder de deken ligt te slapen

In Dutch, it is normal to separate such clauses with a comma, especially when terwijl means while.

Here terwijl means while, showing that two things are happening at the same time.

So the sentence means that:

• Sofie is putting away her pajamas
• while Anna is already sleeping under the blanket

But terwijl can also sometimes have a contrastive meaning, closer to whereas:

• Sofie ruimt op, terwijl Anna niets doet. = Sofie is tidying up, whereas Anna is doing nothing.

In your sentence, the time meaning (while) is the most natural one.

ligt te slapen literally means is lying sleeping, or more naturally in English, is asleep / is lying there asleep.

Dutch often uses posture verbs like:

• liggen = to lie
• zitten = to sit
• staan = to stand

plus te + infinitive to describe how someone is doing something.

So:

• Anna ligt te slapen = Anna is lying down sleeping

This gives a slightly more visual, physical picture than just:

• Anna slaapt = Anna is sleeping

Both are correct, but ligt te slapen emphasizes that she is in a lying position in bed.

It is built like this:

• ligt = finite form of liggen
• te slapen = te
  • infinitive slapen

This pattern is common in Dutch:

• Hij zit te lezen. = He is sitting reading.
• Ze staat te wachten. = She is standing waiting.
• Het kind ligt te slapen. = The child is lying asleep / lying there sleeping.

So liggen te + infinitive expresses both posture and ongoing activity.

Because the clause after terwijl is a subordinate clause, and Dutch subordinate clauses usually push the finite verb toward the end.

Main clause:

• Sofie bergt haar pyjama op.

Subordinate clause:

• terwijl Anna al onder de deken ligt te slapen

That word order is normal Dutch subordinate-clause order.

A rough breakdown:

• terwijl = subordinating conjunction
• Anna = subject
• al onder de deken = other information
• ligt te slapen = verb group at the end

al means already.

So:

• Anna ligt onder de deken te slapen = Anna is sleeping under the blanket.
• Anna ligt al onder de deken te slapen = Anna is already sleeping under the blanket.

It suggests Anna is ahead of Sofie in the bedtime routine. Sofie is still putting away her pajamas, but Anna is already in bed asleep.

onder de deken literally means under the blanket.

It gives a more specific image than just in bed. It tells you Anna is tucked in under the covers.

Some related expressions:

• in bed = in bed
• onder de deken = under the blanket
• onder de dekens = under the blankets/covers

Dutch often uses this kind of concrete expression to create a cozy bedtime image.

Yes, that is possible.

• onder de deken = under the blanket
• onder de dekens = under the blankets / under the covers

Both can sound natural, depending on what you want to emphasize. Onder de dekens is very common in everyday speech, especially when talking about being tucked into bed. Onder de deken is also perfectly fine if you are thinking of one specific blanket.

Yes, that is grammatically fine, but it is slightly different in feel.

• Anna slaapt al onder de deken = Anna is already sleeping under the blanket.
• Anna ligt al onder de deken te slapen = Anna is already lying under the blanket asleep.

The version with ligt te slapen is more descriptive and paints the scene more clearly. It sounds very natural in storytelling or simple narration.

Yes. Depending on the exact meaning, Dutch could also use:

• wegleggen = put away
• opruimen = tidy up
• neerleggen = lay down / put down

But opbergen is especially good when something is being put back in its proper place.

So:

• Sofie bergt haar pyjama op sounds very natural if she is putting it away neatly after getting changed.