Sonen er stengt.

Because sonen is the definite singular form of the noun sone.

• en sone = a zone
• sonen = the zone

In Norwegian, the idea of the is often added to the end of the noun instead of being a separate word. So the -en ending is doing the job that the does in English.

Sone is a common-gender noun in Bokmål.

Its basic forms are:

• en sone = a zone
• sonen = the zone
• soner = zones
• sonene = the zones

So in your sentence, sonen is just the regular definite singular form.

Stengt comes from the verb å stenge, which means to close, to shut, or to block off depending on context.

In Sonen er stengt, stengt is the past participle used in a way that describes a state:

• er = is
• stengt = closed / shut / blocked off

So the sentence is describing the condition of the zone, not the action of closing it right now.

This is a very useful distinction:

• er stengt = is closed, is in a closed state
• blir stengt = is being closed, gets closed

So:

• Sonen er stengt means the zone is already closed.
• Sonen blir stengt means the zone is getting closed or will be closed as part of an action/process.

English often uses is closed for the state, and Norwegian usually uses er stengt in the same way.

After er in this kind of sentence, Norwegian normally uses stengt to describe the resulting state.

Compare:

• en stengt dør = a closed door
• de stengte dørene = the closed doors
• dørene er stengt = the doors are closed

So stengte is common when the word comes before a noun in certain forms, but stengt is the normal form here after er.

Yes. Sone is basically the Norwegian equivalent of zone.

Depending on context, it can mean things like:

• a geographic zone
• a security zone
• a traffic zone
• a restricted area
• a tariff or fare zone

So if you already know the meaning from context, the form of the word should feel fairly familiar to an English speaker.

No. That would be sønnen, with ø and a different spelling.

• sonen = the zone
• sønnen = the son

This is an important difference, because for English speakers the two can look similar at first. In Norwegian, the vowel matters a lot.

Sometimes, but stengt is usually the more natural choice for things like areas, roads, shops, borders, websites, and services that are not open or available.

Very roughly:

• stengt = closed to access, use, or entry
• lukket = closed in a more literal or structural sense, or closed-off in a more formal/abstract sense

For a zone or area, stengt is usually the better and more idiomatic word.

For a yes/no question, you switch the order of the verb and the subject:

• Sonen er stengt. = The zone is closed.
• Er sonen stengt? = Is the zone closed?

That verb-before-subject pattern is very common in Norwegian yes/no questions.

A rough English-friendly guide would be:

SOO-nen er stengt

A few details:

• sonen has stress on the first syllable
• the o is long here, roughly like oo in food
• er is short, roughly ehr
• in stengt, the ng sounds like the ng in sing, followed by a clear t

So a rough approximation is:

SOO-nen ehr stengt