Als het ’s avonds afkoelt, gaat Sofie van het balkon naar binnen en zet zij de kachel hoger.

Als means if or when in this kind of sentence. It introduces a subordinate clause:

Als het ’s avonds afkoelt = When/If it cools down in the evening

In everyday Dutch, als is very common for situations like this. Because it starts a subordinate clause, the verb in that clause goes to the end: afkoelt.

Because Als het ’s avonds afkoelt is a subordinate clause.

In Dutch subordinate clauses, the conjugated verb usually goes to the end. So:

• het koelt af = it cools down
• als het afkoelt = when/if it cools down

Here, the separable verb afkoelen becomes afkoelt at the end of the clause.

’s avonds means in the evening.

The ’s is an old shortened form that also appears in expressions like:

• ’s morgens = in the morning
• ’s middags = in the afternoon
• ’s nachts = at night

For a learner, the important thing is to recognize ’s avonds as a fixed time expression meaning in the evening. You do not need to analyze the historical grammar to use it correctly.

Afkoelen is a separable verb.

Its full infinitive is afkoelen = to cool down.
In a main clause, the prefix often separates:

• Het koelt af.

But in a subordinate clause, the parts stay together:

• als het afkoelt

That is why you see afkoelt as one word here.

Because the sentence begins with the subordinate clause:

Als het ’s avonds afkoelt, ...

After that opening element, Dutch main clauses follow the verb-second rule. That means the finite verb comes first in the main clause, before the subject:

• Als het ’s avonds afkoelt, gaat Sofie ...
• not Als het ’s avonds afkoelt, Sofie gaat ...

This is very normal Dutch word order.

Dutch often expresses movement very explicitly:

• van het balkon = from the balcony
• naar binnen = inside / to the inside

So gaat Sofie van het balkon naar binnen literally describes movement from the balcony to inside.

This is very natural in Dutch. English often says simply goes inside, but Dutch likes to spell out both the starting point and the destination.

Both can occur in Dutch, but naar binnen gaan strongly emphasizes movement inward: to go inside.

Compare:

• binnen zijn = to be inside
• naar binnen gaan = to go inside

So naar binnen works well here because Sofie is moving from one place to another.

This is again because of the verb-second rule in the main clause.

After the opening subordinate clause, the main clause starts with the verb:

• gaat Sofie ...
• en zet zij ...

The subject follows the verb: Sofie, zij.

Also, zij here is just a slightly more formal or written style than ze. In speech, many people would say:

... en zet ze de kachel hoger.

Dutch often avoids repeating the noun if the subject is still clear. So after Sofie is introduced, it is natural to continue with zij:

... gaat Sofie ... en zet zij ...

You could repeat Sofie, but using the pronoun sounds smoother.

In less formal spoken Dutch, ze is more common than zij.

De kachel hoger zetten means to turn the heater up.

Literally:

• de kachel = the heater/stove
• hoger = higher
• zetten = to put/set

Dutch often uses hoger/lager zetten for adjusting levels:

• de verwarming hoger zetten = turn the heating up
• de radio zachter zetten = turn the radio down/softer
• de temperatuur lager zetten = lower the temperature

So hoger here refers to increasing the setting.

Because the sentence is about changing a setting, not directly describing the result.

• hoger zetten = set it higher
• warmer maken = make it warmer

Dutch commonly uses hoger and lager for controls, levels, and settings. With a heater, de kachel hoger zetten is the normal expression.

Not necessarily. In modern usage, kachel can refer to a heater or heating device in general, depending on context.

So in this sentence, it most naturally means something like the heater rather than specifically a traditional stove.

Because the sentence has one subordinate clause followed by a coordinated main clause:

• Als het ’s avonds afkoelt = subordinate clause
• gaat Sofie van het balkon naar binnen = first main clause
• en zet zij de kachel hoger = second coordinated main clause

The conjunction en joins two main-clause actions that both happen in that situation. You do not need a second als.

Yes, wanneer is possible:

Wanneer het ’s avonds afkoelt, ...

But als is more common and more natural in many everyday sentences like this. Wanneer can sound a bit more formal or more specific depending on context.

A learner would often notice these key points:

1. Subordinate clause word order
   Als het ’s avonds afkoelt
   The verb goes to the end.

2. Verb-second in the main clause
   ... gaat Sofie ...
   After the subordinate clause, the finite verb comes first in the main clause.

3. Separable verb
   afkoelen becomes afkoelt in the subordinate clause.

4. Fixed time expression
   ’s avonds = in the evening.

5. Natural motion expression
   van het balkon naar binnen = from the balcony to inside.

6. Common Dutch way to express adjusting a setting
   de kachel hoger zetten = turn the heater up.