Når hun er i tvivl, stiller hun læreren et spørgsmål.

Når means when in the sense of something that happens generally or repeatedly.

So Når hun er i tvivl means When she is in doubt / When she is unsure.

A learner may compare it with:

• når = when, for repeated or general situations
• da = when, for a specific event in the past
• hvis = if

In this sentence, når fits because it describes a usual pattern: whenever she is unsure, she asks a question.

This is because Danish uses verb-second word order in main clauses.

When a different element comes first—here, the whole subordinate clause Når hun er i tvivl—the finite verb in the main clause must come before the subject:

• Når hun er i tvivl, stiller hun læreren et spørgsmål.

If the sentence started directly with the subject, it would be:

• Hun stiller læreren et spørgsmål, når hun er i tvivl.

So the inversion stiller hun is completely normal after an opening clause like this.

Yes, være i tvivl is a very common fixed expression in Danish.

Literally, it means something like to be in doubt, but in natural English it often means:

• to be unsure
• to be uncertain
• to have doubts

So hun er i tvivl means she is uncertain or not sure about something.

You should learn være i tvivl as a chunk, because Danish uses it very often.

Yes, stille often means place, put, or set, but in the expression stille et spørgsmål, it means ask a question.

This is a standard collocation in Danish:

• stille et spørgsmål = ask a question

So even though the literal image is a bit like pose a question, the natural meaning is simply ask a question.

Because læreren is the indirect object and et spørgsmål is the direct object.

In Danish, when both objects are noun phrases, the usual order is:

• indirect object + direct object

So:

• hun stiller læreren et spørgsmål
• literally: she asks the teacher a question

This is similar to English patterns like:

• She gives the teacher a book
• She asks the teacher a question

You can also sometimes rephrase with a preposition, but this order is the normal one here.

Because spørgsmål is a neuter noun in Danish.

Danish nouns have two grammatical genders:

• en-words (common gender)
• et-words (neuter)

Since spørgsmål is neuter, you say:

• et spørgsmål = a question

And in the definite form:

• spørgsmålet = the question

Læreren means the teacher, while en lærer means a teacher.

The sentence uses læreren because it refers to a specific teacher, probably one already known from context, such as her teacher in class.

So:

• læreren = the teacher
• en lærer = a teacher

Both are grammatically possible in different contexts, but they mean slightly different things.

Yes. Both instances of hun refer to the same person.

Danish repeats the subject in the main clause even if it is already understood from the subordinate clause. So you cannot leave it out here.

That means:

• Når hun er i tvivl, stiller hun læreren et spørgsmål.

is correct, but Danish does not drop the second hun the way some learners might expect.

Yes:

• Hun stiller læreren et spørgsmål, når hun er i tvivl.

This means the same thing.

The difference is mostly one of emphasis and structure:

• Starting with Når hun er i tvivl gives the condition or situation first.
• Starting with Hun stiller læreren et spørgsmål gives the action first.

Both are natural Danish.

Yes, the comma separates the subordinate clause from the main clause:

• Når hun er i tvivl, stiller hun læreren et spørgsmål.

The part beginning with når is a subordinate clause, and the rest is the main clause.

In learner materials, this comma is very helpful because it clearly shows where the clause boundary is, and that also helps explain the inversion stiller hun in the main clause.