Hvis appen lukker under mødet, må jeg genåbne den med det samme.

Because Danish main clauses normally follow verb-second word order. When the sentence begins with Hvis appen lukker under mødet, that whole hvis-clause takes the first position, so the finite verb of the main clause comes next: må, then the subject jeg.

Without the opening clause, the basic word order would be Jeg må genåbne den med det samme.

Danish usually shows the by adding an ending to the noun.

• en app = an app
• appen = the app

So -en is the definite ending here.

Because app is a common-gender noun: en app. When you refer back to an en-word, you normally use den.

• en app ... den
• et program ... det

So den matches the gender of appen.

Yes. Under can be used in a time sense as well as a physical sense.

Here, under mødet means during the meeting, not physically underneath anything. This is very normal Danish usage.

The definite form mødet shows that this is a specific meeting, usually one already known from the context.

• under mødet = during the meeting
• under et møde = during a meeting

So the sentence is talking about a particular meeting, not just any meeting.

In this sentence, yes. Appen is the subject, so appen lukker means the app closes.

Danish lukke can be used both ways:

• Appen lukker = the app closes
• Jeg lukker appen = I close the app

So the same verb can be intransitive or transitive, depending on the sentence.

Yes. That is one reason må can be confusing for learners. It can express either permission or necessity, and the context tells you which meaning is intended.

For example:

• Du må gå nu can mean you are allowed to go now.
• Jeg må gå nu often means I have to go now.

In this sentence, use the meaning shown in your translation.

Gen- is a prefix meaning re- or again, so genåbne means reopen.

Also, after modal verbs like må, kan, skal, vil, Danish uses the infinitive without at.

So:

• må genåbne = correct
• må at genåbne = not correct

Because Danish often uses the present tense for future or conditional situations, especially after hvis.

This is similar to English:

• If the app closes...
• not usually If the app will close...

So lukker and må are perfectly normal here.

Hvis is used for something conditional or uncertain: it may happen, but it may not.

Når is used when something is expected to happen, or for repeated situations.

Since the sentence treats the app closing as a possibility, hvis is the right choice.

Yes. It is best learned as a fixed expression meaning immediately or right away.

You do not need to translate each word separately. As a whole, med det samme is a very common natural Danish phrase.

The comma separates the opening hvis-clause from the main clause.

• Hvis appen lukker under mødet = subordinate clause
• må jeg genåbne den med det samme = main clause

So the comma marks the boundary between those two parts of the sentence.