När strömmen försvann under kvällen blev hela lägenheten tyst.

Here När means when and introduces a time clause:

• När strömmen försvann ... = When the power went out ...

A useful extra point: när can also mean when in a question:

• När kommer du? = When are you coming?

And in some contexts it can mean whenever, depending on the sentence.

This is because of Swedish V2 word order.

The sentence starts with a subordinate clause:

• När strömmen försvann under kvällen

After that, the main clause begins, and in Swedish the finite verb must come in the second position of the main clause. So you get:

• ... blev hela lägenheten tyst

Not:

• ... hela lägenheten blev tyst

That version would be wrong in this sentence structure.

If you put the main clause first, then the normal order returns:

• Hela lägenheten blev tyst när strömmen försvann under kvällen.

Because Swedish usually adds the definite article to the end of the noun.

So:

• ström = power/current
• strömmen = the power

and

• lägenhet = apartment
• lägenheten = the apartment

This is one of the biggest differences from English: instead of a separate word like the, Swedish often uses a suffix.

In this sentence, strömmen means the electricity / the power supply.

The basic noun ström can mean things like:

• current
• flow
• electric current

But in everyday Swedish, when talking about a home and something stopping, strömmen usually means the power.

So strömmen försvann is understood as:

• the power went out
• the electricity cut out

Försvann is the past tense of försvinna, which basically means disappear.

So literally:

• strömmen försvann = the power disappeared

In natural English, you would usually say:

• the power went out

Yes, this is natural Swedish. It is idiomatic and understandable.

Other common Swedish ways to talk about a power outage include:

• Strömmen gick.
• Det blev strömavbrott.

But strömmen försvann works well and is clear.

Under kvällen means during the evening or at some point in the course of the evening.

It gives a sense of time span or period:

• something happened sometime during that evening

By contrast, på kvällen often means in the evening in a more general time-of-day sense.

So:

• under kvällen = during the evening
• på kvällen = in the evening

In this sentence, under kvällen fits because it places the event within the evening period.

Because blev means became, while var means was.

So:

• blev hela lägenheten tyst = the whole apartment became quiet
• var hela lägenheten tyst = the whole apartment was quiet

Using blev highlights a change of state: first there was power, then the power disappeared, and as a result the apartment became quiet.

That is why blev is the better choice here.

Because hela often works with a definite noun directly.

So Swedish says:

• hela lägenheten = the whole apartment
• hela huset = the whole house
• hela dagen = the whole day

You do not normally add den here.

This is different from many other adjective structures in Swedish, where you often get double definiteness, such as:

• den stora lägenheten = the big apartment

But hela behaves differently.

Because the subject is singular.

In predicative position (after verbs like bli), tyst stays singular:

• Lägenheten blev tyst. = The apartment became quiet.
• Huset blev tyst. = The house became quiet.

But in the plural, you use tysta:

• Lägenheterna blev tysta. = The apartments became quiet.

So here hela lägenheten is singular, which is why tyst is correct.

Yes. It is natural.

Even though an apartment itself is not literally a person or thing that makes noise, both Swedish and English can use this kind of expression naturally to describe the overall environment.

So hela lägenheten blev tyst means that everything in the apartment went quiet:

• no humming appliances
• no electrical sounds
• a sudden silence in the home

It is a normal and idiomatic way to describe the situation.