Jeg må installere appen igen, hvis den ikke virker.

Here, må means must, have to, or need to.

Danish må can also sometimes mean may or be allowed to, so it can be a tricky word. In this sentence, though, the context makes necessity the natural meaning:

Jeg må installere appen igen
= I have to reinstall the app

Because må is a modal verb.

After Danish modal verbs, the next verb is used in the infinitive without at:

• jeg må installere
• jeg kan installere
• jeg skal installere

So jeg må at installere would be incorrect.

In Danish, the definite article is usually added to the end of the noun.

• en app = an app
• appen = the app

So appen is the definite form of app.

This is different from English, where the is a separate word.

Because app is a common gender noun in Danish:

• en app
• therefore: den

If it were a neuter noun (et noun), you would use det instead.

So:

• appen → den
• hvis den ikke virker = if it doesn’t work

Because hvis den ikke virker is a subordinate clause.

In Danish, words like ikke often come before the finite verb in subordinate clauses:

• hvis den ikke virker

But in a main clause, ikke usually comes after the finite verb:

• Den virker ikke = It doesn’t work

So the word order changes because hvis introduces a subordinate clause.

Virker comes from virke, and here it means works or functions.

So:

• den virker = it works
• den ikke virker = it doesn’t work

This verb is very common for technology, machines, software, buttons, tools, and systems.

Igen means again.

In this sentence, it naturally comes after the object:

• installere appen igen

That means install the app again.

This word order is very normal in Danish. The sentence sounds natural as written.

Yes.

Literally, it means install the app again, but in normal English that often becomes reinstall the app.

Danish can also express this with a single verb:

• geninstallere appen = reinstall the app

So these are very close in meaning:

• installere appen igen
• geninstallere appen

You could, but the meaning changes a little.

• må often suggests necessity: I have to / I need to
• skal often suggests plan, instruction, or obligation: I am supposed to / I will have to

So:

• Jeg må installere appen igen = I need to reinstall the app
• Jeg skal installere appen igen = I am going to / am supposed to reinstall the app

In your sentence, må fits well because the app not working creates the need.

Because Danish often uses the present form or a modal verb to talk about what needs to happen next.

Jeg må installere appen igen can naturally refer to a present necessity with a future result.

Danish does not need a special future form in cases like this. Context usually makes the time clear.

Hvis means if and introduces a condition:

• hvis den ikke virker = if it doesn’t work

So the whole sentence means that reinstalling the app depends on that condition.

It is the normal word for if in conditional sentences.

Yes. You can put the if-clause first:

Hvis den ikke virker, må jeg installere appen igen.

That means the same thing.

Notice that when the subordinate clause comes first, the main clause still follows Danish V2 word order, so må comes before jeg:

• Hvis den ikke virker, må jeg ...

not

• Hvis den ikke virker, jeg må ...

Not quite.

In a condition like this, hvis is the correct word:

• hvis den ikke virker = if it doesn’t work

But Danish also uses om for whether/if in indirect questions:

• Jeg ved ikke, om den virker = I don’t know whether it works

So:

• hvis = conditional if
• om = whether / if in indirect-question type meanings