Døren vil blive lukket, når mødet begynder.

Because Danish usually puts the definite article at the end of the noun.

• dør = door
• døren = the door

So -en here is the equivalent of English the.
The same thing happens with mødet:

• møde = meeting
• mødet = the meeting

This is one of the biggest differences from English, where the comes before the noun.

It is forming a future passive construction.

Breakdown:

• vil = marks future, like will
• blive = become / be
• lukket = past participle of lukke, meaning closed

So:

• vil blive lukket = will be closed

This is similar to English will be closed, but Danish often uses blive to form the passive.

Lukket is the past participle of the verb lukke.

• infinitive: lukke = to close
• past participle: lukket = closed

In this sentence, it is used together with blive to make a passive:

• blive lukket = be closed

So lukket is not acting as the main finite verb by itself. It is part of the passive verb phrase.

The passive is used because the sentence focuses on the door, not on who closes it.

Compare:

• Nogen vil lukke døren, når mødet begynder.
  = Someone will close the door when the meeting begins.

• Døren vil blive lukket, når mødet begynder.
  = The door will be closed when the meeting begins.

In the passive version, the person doing the action is either unknown or unimportant.

Når is used for when in cases like this, especially for something that is expected to happen.

Here it introduces the time clause:

• når mødet begynder = when the meeting begins

For English speakers, it helps to think of når as the normal word for when in future or repeated situations.

It is different from words like:

• hvis = if
• da = often when about a specific event in the past

So in this sentence, når is the natural choice.

Because Danish, like English, often uses the present tense in time clauses referring to the future.

So Danish says:

• når mødet begynder
  literally: when the meeting begins

not:

• når mødet vil begynde

This works much like English, where we say:

• The door will be closed when the meeting begins

not normally:

• when the meeting will begin

So the future meaning is already clear from the structure, and Danish does not need vil inside that når clause.

Because når mødet begynder is a subordinate clause, introduced by når.

In Danish subordinate clauses, the word order is usually:

• conjunction + subject + finite verb

So:

• når mødet begynder

not:

• når begynder mødet

English speakers sometimes expect inversion because Danish often has verb-second word order in main clauses, but that does not normally happen inside subordinate clauses like this one.

Yes, very often.

In everyday Danish, the present tense is frequently used to talk about the future when the context already makes the timing clear.

So both are possible:

• Døren vil blive lukket, når mødet begynder.
• Døren bliver lukket, når mødet begynder.

The version with vil sounds a bit more explicitly future-oriented. The version without vil can sound slightly more natural in many everyday contexts.

Yes. If the subordinate clause comes first, the main clause follows normal Danish verb-second order.

So you can say:

• Når mødet begynder, vil døren blive lukket.

Notice what happens in the main clause:

• not døren vil blive lukket
• but vil døren blive lukket

That is because the first position is already occupied by Når mødet begynder, so the finite verb vil must come next.

Not always.

Vil can also express willingness, intention, or something like want to depending on context. But in this sentence, it is most naturally understood as a future marker.

For example:

• Jeg vil hjælpe dig.
  This can mean I will help you or I want to help you, depending on context.

But in:

• Døren vil blive lukket, når mødet begynder

the meaning is clearly future, not desire. A door cannot really want to be closed, so the future reading is the natural one.