Publikum klapper, når forestillingen er slut.

Publikum means the audience or an audience, but it is often used without an article in Danish when speaking about the audience as a group in a general or situational way.

So:

• Publikum klapper = The audience applauds / People in the audience clap
• Publikummet klapper is also possible, but it sounds a bit more specifically like that particular audience

In short, bare publikum is very natural here.

Grammatically, publikum is usually treated as a singular collective noun. It refers to many people, but as one group.

That is similar to English the audience.

However, in this sentence, you do not really notice singular vs. plural from the verb, because Danish present-tense verbs do not change by person or number:

• jeg klapper
• du klapper
• publikum klapper
• de klapper

The form klapper stays the same.

Klapper is the present tense of at klappe, which literally means to clap.

So Publikum klapper means:

• The audience claps
• or more naturally in English, The audience applauds

Danish often uses at klappe where English might prefer to applaud.

Here når means when.

It is used for:

• something that happens generally
• something that happens repeatedly
• or something expected to happen in the future

So Publikum klapper, når forestillingen er slut means something like:

• The audience applauds when the performance is over
• The audience will applaud when the performance ends

A useful contrast:

• når = when, for repeated/general/future situations
• da = when, for a specific event in the past

Danish often uses the present tense for things that are:

• generally true
• habitual
• or expected in the future

So this sentence is not necessarily about what is happening right now. It describes what normally happens:

• Publikum klapper = The audience applauds
• når forestillingen er slut = when the performance is over

English can also do this: People clap when the show is over.

Because forestillingen is the definite singular form of forestilling.

• en forestilling = a performance / a show
• forestillingen = the performance / the show

Danish usually adds the definite article to the end of the noun:

• bil → bilen
• bog → bogen
• forestilling → forestillingen

So the -en at the end means the.

Er slut means is over or has ended.

In this sentence, slut is functioning like an adjective after er:

• Forestillingen er slut = The performance is over

This is a very common Danish expression. You could think of it as idiomatic.

A close alternative is er færdig, but er slut is especially natural for events, performances, films, meetings, and similar things.

Because når forestillingen er slut is a subordinate clause, and Danish subordinate clauses do not use the normal main-clause verb-second pattern.

Compare:

• Main clause: Forestillingen er slut
• Subordinate clause: når forestillingen er slut

So after når, the subject forestillingen comes before the verb er.

That is one of the most important Danish word-order patterns to learn.

Yes, absolutely. That is a very natural alternative.

Both are correct:

• Publikum klapper, når forestillingen er slut.
• Når forestillingen er slut, klapper publikum.

The difference is mostly about focus:

• starting with Publikum klapper emphasizes the audience’s action
• starting with Når forestillingen er slut emphasizes the time condition first

Notice the word order in the second version: after the subordinate clause comes klapper publikum, not publikum klapper. That is because the main clause follows the Danish verb-second rule.

Because når forestillingen er slut is a subordinate clause, and Danish normally places a comma before subordinate clauses.

So:

• Publikum klapper, når forestillingen er slut.

This comma is standard Danish punctuation.

Not only. Forestilling can mean several kinds of performance or show, depending on context, such as:

• a theatre performance
• a stage show
• sometimes a screening or presentation-like event

In this sentence, it most naturally means the performance or the show.

Yes.

Depending on context, it can mean:

• In general, the audience applauds when the performance is over
• or This time, the audience will applaud when the performance is over

Danish present tense often covers both meanings, and context tells you which one is intended.