Anna zit op het balkon te lezen, terwijl de kachel binnen nog aanstaat.

Dutch often uses a posture verb like zitten (to sit), staan (to stand), or liggen (to lie) plus te + infinitive to describe what someone is doing.

So:

• Anna leest op het balkon = Anna reads / is reading on the balcony
• Anna zit op het balkon te lezen = Anna is sitting on the balcony reading

This version gives a more vivid picture: Anna is not just reading; she is sitting there and reading.

In everyday Dutch, this is very common.

In zit ... te lezen, the te + infinitive part expresses the activity that happens while the subject is in that position.

So:

• zit te lezen = is sitting reading
• literally, something like sits to read, but that is not how you should translate it into English

Other examples:

• Hij staat te praten. = He is standing talking.
• De kat ligt te slapen. = The cat is lying sleeping.

So in this sentence, te lezen is not a separate purpose phrase like in order to read. It is part of a fixed pattern describing an ongoing action.

Yes. Dutch also has another common progressive construction:

• Anna is op het balkon aan het lezen.

This also means Anna is reading on the balcony.

The difference is roughly:

• zit te lezen emphasizes her position while doing it
• is aan het lezen emphasizes the action as ongoing

Both are natural, but zit te lezen feels especially suitable because someone reading on a balcony is very likely sitting.

Dutch uses op with balkon:

• op het balkon = on the balcony

This matches English fairly well. Dutch sees a balcony as a kind of surface/platform you are on.

Examples:

• op het balkon
• op het dak = on the roof
• op het terras = on the terrace

So op het balkon is simply the normal Dutch choice.

Terwijl usually means while.

It introduces a subordinate clause, and that matters because subordinate clauses in Dutch have different word order from main clauses.

Here:

• Anna zit op het balkon te lezen = main clause
• terwijl de kachel binnen nog aanstaat = subordinate clause

So terwijl connects the two ideas:

• Anna is reading on the balcony
• at the same time, the heater inside is still on

Because terwijl creates a subordinate clause, and in Dutch subordinate clauses the finite verb typically goes to the end.

The verb here is aanstaan (to be on, for devices, lights, heating, etc.).

In a main clause, you would normally see it split:

• De kachel staat nog aan.

But after terwijl, it goes to the end as one unit:

• terwijl de kachel binnen nog aanstaat

This is a very important Dutch pattern.

Compare:

• Het licht is aan.
• Ik zie dat het licht aan is.
  or with separable verbs:
• De televisie staat aan.
• Ik zie dat de televisie aanstaat.

Aanstaan is a separable verb.

Its parts are:

• aan = particle
• staan = verb

In a normal main clause, separable verbs split:

• De kachel staat aan.

In infinitives and many subordinate clauses, they stay together:

• aanstaan
• ... terwijl de kachel aanstaat
• ... omdat de kachel al uren aanstaat

This is one of the most common things learners notice in Dutch word order.

Here binnen means inside / indoors.

It tells you where the heater is:

• Anna is outside, on the balcony
• the heater is inside

So de kachel binnen means something like the heater inside.

Dutch often uses binnen as an adverb like this:

• Het is binnen warm. = It is warm inside.
• De kinderen spelen binnen. = The children are playing indoors.

In your sentence, it helps create the contrast between outside on the balcony and inside where the heater is on.

Here nog means still.

So:

• de kachel staat nog aan = the heater is still on

It suggests that maybe you would expect it to be off already, but it remains on.

This is a very common use of nog:

• Hij slaapt nog. = He is still sleeping.
• Ben je nog thuis? = Are you still at home?

Be careful: nog can have several meanings depending on context, such as still, yet, another, or more.

Because kachel is a de-word in Dutch:

• de kachel = the heater / stove

This is something you usually have to learn with the noun.

There is no strong logic you can always rely on, so it is best to memorize nouns with their article:

• de kachel
• het balkon

That is why the sentence has:

• op het balkon
• de kachel

Because balkon is a het-word:

• het balkon = the balcony

Again, this is lexical information you need to learn with the noun.

Useful forms:

• het balkon
• een balkon
• op het balkon

In normal writing, yes, a comma before a terwijl clause is very common and appropriate here.

The comma helps separate:

1. the main clause
   Anna zit op het balkon te lezen
2. the subordinate clause
   terwijl de kachel binnen nog aanstaat

It makes the sentence easier to read and reflects the structure clearly.

Often yes, but not always in exactly the same way as English while.

In many cases terwijl simply expresses two things happening at the same time:

• Zij kookt terwijl hij de tafel dekt. = She cooks while he sets the table.

Sometimes it can also suggest contrast:

• Hij rust uit, terwijl zij hard werkt. = He is resting, while she is working hard.

In your sentence, there is both simultaneity and a bit of contrast: Anna is outside reading, while the heater inside is still on.

The sentence has two parts:

1. Anna zit op het balkon te lezen

   • Subject: Anna
   • Finite verb: zit
   • Place: op het balkon
   • Infinitive phrase: te lezen

2. terwijl de kachel binnen nog aanstaat

   • Subordinator: terwijl
   • Subject: de kachel
   • Adverb: binnen
   • Adverb: nog
   • Finite verb at the end: aanstaat

So a very rough pattern is:

• Main clause: Subject + finite verb + rest
• Subordinate clause: conjunction + subject + rest + finite verb

That contrast is one of the key features of Dutch syntax.

Yes, that is grammatically correct.

It would simply mean:

• Anna is reading on the balcony, while the heater inside is still on.

Compared with Anna zit op het balkon te lezen, it is a bit less visual.
The original sentence specifically paints Anna as sitting there reading, which sounds natural and descriptive.

So both are correct, but the original version is a little richer in detail.