Wanneer die kinders slaap, is die huis rustig.

Because Afrikaans uses die as the definite article for both singular and plural nouns.

So:

• die huis = the house
• die kinders = the children

Unlike languages such as German or Dutch, Afrikaans does not change the definite article for gender, and it also does not have a separate plural form of the.

The plural ending -s shows that kinders is plural.

• kind = child
• kinders = children

This is helpful for English speakers because English has the irregular plural children, but Afrikaans uses the more regular pattern here.

In Afrikaans, verbs usually do not change form for different subjects in the present tense.

So you get:

• ek slaap = I sleep
• jy slaap = you sleep
• die kind slaap = the child sleeps
• die kinders slaap = the children sleep

This is much simpler than English, where you have a difference like sleep vs sleeps.

This is a very common Afrikaans word-order pattern.

The basic main clause is:

• Die huis is rustig. = The house is calm/quiet.

But when the sentence starts with another element, here the time clause Wanneer die kinders slaap, the finite verb of the main clause usually comes immediately after that opening element.

So:

• Wanneer die kinders slaap, is die huis rustig.

This is often called inversion. English does not usually do this, so it can feel unfamiliar.

The comma separates the opening time clause from the main clause.

• Wanneer die kinders slaap = time clause
• is die huis rustig = main clause

In standard written Afrikaans, a subordinate clause placed before the main clause is normally followed by a comma.

Because rustig is being used after the verb is as a predicate adjective.

In Afrikaans, adjectives usually stay in their basic form after verbs like is:

• Die huis is rustig.

But before a noun, an adjective often takes -e:

• ’n rustige huis = a calm/quiet house

So:

• predicative position: die huis is rustig
• before a noun: die rustige huis or ’n rustige huis

Yes. That is also correct.

Both of these are natural:

• Wanneer die kinders slaap, is die huis rustig.
• Die huis is rustig wanneer die kinders slaap.

The difference is mainly focus:

• Starting with Wanneer die kinders slaap gives a little more emphasis to the time setting.
• Starting with Die huis is rustig sounds a bit more straightforward and neutral.

Wanneer introduces a time clause. It means when and tells you at what time or under what circumstances the main clause is true.

So in this sentence, wanneer die kinders slaap tells us when the house is quiet.

It is functioning much like English when in:

• When the children sleep, the house is quiet.

Sometimes yes, but wanneer is a very clear and safe choice for when in a time clause like this.

Very roughly:

• wanneer = when
• as can sometimes mean when in everyday Afrikaans, but it is also very commonly if

Because of that, learners often find wanneer easier and less ambiguous in sentences like this. So the sentence as given is a good model to follow.

Not necessarily. Rustig can also mean things like:

• calm
• peaceful
• restful
• tranquil

So Die huis is rustig can suggest not just lack of noise, but also a calm, peaceful atmosphere. The exact English translation depends on context.