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

Because als introduces a subordinate clause. In Dutch subordinate clauses, the conjugated verb usually goes to the end.

So:

• Als = if / when
• het 's avonds = it / in the evening
• afkoelt = cools down

This is a very common Dutch pattern:

• Als het regent, blijf ik thuis.
• Omdat hij moe is, gaat hij slapen.

In both cases, the verb of the subordinate clause comes at the end.

's avonds means in the evening or in the evenings, depending on context.

The 's is an old shortened form of des, from older Dutch. You do not need to analyze it deeply as a beginner; it is best learned as a fixed time expression.

Common examples:

• 's morgens = in the morning
• 's middags = in the afternoon
• 's avonds = in the evening
• 's nachts = at night

So het 's avonds afkoelt means it cools down in the evening.

Because the sentence begins with the subordinate clause Als het 's avonds afkoelt. After that, the main clause follows, and Dutch keeps the finite verb in second position.

So the structure is:

• first position: Als het 's avonds afkoelt
• second position: gaat
• then the subject: Sofie

That gives:

• Als het 's avonds afkoelt, gaat Sofie ...

If you started directly with Sofie, then you would say:

• Sofie gaat van het balkon naar binnen ...

This is a key Dutch word-order rule: when something other than the subject comes first, the verb usually comes before the subject.

Koelen means to cool or to make cool.
Afkoelen means to cool down, often becoming cooler gradually.

In this sentence, afkoelt is used because the temperature is dropping:

• Het koelt af. = It is cooling down.

The prefix af- often adds the sense of a change or completion. Here it helps create the idea of becoming cooler rather than just cooling in a more general sense.

This is because Dutch separable verbs behave differently depending on clause type.

Afkoelen is a separable verb:

• in a main clause: Het koelt af.
• in a subordinate clause: ... als het afkoelt.

So:

• Main clause: prefix and verb split
• Subordinate clause: prefix and verb stay together

That is why this sentence has Als het 's avonds afkoelt and not Als het 's avonds koelt af.

This shows movement from one place to another:

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

Dutch often expresses this movement very clearly with two parts:

• where you are coming from
• where you are going

So gaat Sofie van het balkon naar binnen means she moves from the balcony into the house/apartment.

Both can appear in Dutch, but naar binnen gaan is a very common way to express movement inward: to go inside.

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

So:

• binnen often describes location
• naar binnen emphasizes direction or movement

That is why gaat ... naar binnen fits naturally here.

This is again the normal Dutch main-clause word order after coordination.

The first main clause is:

• gaat Sofie van het balkon naar binnen

Then it continues with en plus another main clause:

• zet zij de kachel hoger

In this second clause, the verb zet comes first because the conjunction en does not count as the first sentence element in the same way a normal adverbial phrase does. Dutch often keeps the usual verb-second structure:

• ... en zet zij de kachel hoger

You could also very commonly hear en ze zet de kachel hoger in everyday speech.

Both zij and ze can mean she.

• ze is the unstressed, more common everyday form
• zij is the stressed or more formal form

In writing, especially in slightly careful or neutral style, zij is perfectly normal. It can also make the sentence sound a bit clearer or more balanced.

So here:

• zet zij de kachel hoger

is grammatically fine and slightly more formal than:

• zet ze de kachel hoger

Literally, it means to set the heater higher.

In natural English, that usually corresponds to:

• turn the heater up
• turn up the heat

Dutch often uses hoger zetten with devices or settings:

• de verwarming hoger zetten = turn the heating up
• de radio harder zetten = turn the radio louder
• de temperatuur lager zetten = set the temperature lower

So hoger is functioning like higher/up with the setting.

Because hoger here does not need an explicit comparison. It means to a higher level than before.

Dutch, like English, can use a comparative form without stating the second part:

• Zet de verwarming hoger. = Turn the heating up.
• Kan het zachter? = Can it be quieter?
• Ik wil het warmer. = I want it warmer.

So hoger means higher than it is now.

Because Dutch nouns have different grammatical articles:

• het balkon
• de kachel

Unfortunately, this usually has to be learned with each noun. There are some patterns, but many nouns simply need to be memorized with their article.

A good habit is to learn nouns as a unit:

• het balkon
• de kachel
• de avond
• het huis

That helps you later with pronouns, adjectives, and plurals too.

In this sentence, als is best understood as when in a repeated or general situation:

• Als het 's avonds afkoelt ... = When it gets cooler in the evening ...

Dutch als can mean if or when, depending on context.

Here it does not sound like an uncertain one-time possibility. It sounds more like a typical situation: whenever the evening gets cooler, Sofie goes inside and turns up the heater.

So a learner should know that als often covers both ideas.

Yes, a similar idea could be expressed that way, but the nuance is slightly different.

• het koelt af = it cools down
• het wordt kouder = it gets colder

Both are natural.
Afkoelen emphasizes the process of cooling down.
Kouder worden emphasizes becoming colder.

So:

• Als het 's avonds afkoelt ...
• Als het 's avonds kouder wordt ...

are both possible, with a small difference in feel rather than a major difference in meaning.