Jeg er i tvivl om, hvilken bog jeg skal tage med til universitetet.

Yes. At være i tvivl om is a very common fixed expression in Danish. It means to be unsure about, to be in doubt about, or not to know for certain.

So:

• Jeg er i tvivl om ... = I’m unsure about ...
• Hun er i tvivl om svaret. = She is unsure about the answer.

It is often followed by:

• a noun: i tvivl om svaret
• a clause: i tvivl om, hvilken bog jeg skal tage med

This is one of the most natural ways in Danish to express uncertainty.

Because om is the preposition that normally goes with i tvivl in this expression.

So Danish says:

• i tvivl om noget = in doubt about something

Examples:

• Jeg er i tvivl om det.
• Vi er i tvivl om, hvad vi skal gøre.

Even though English might use about, Danish uses om here. This is something you mostly just learn as part of the whole expression at være i tvivl om.

Because everything after the comma is a subordinate clause, more specifically an indirect question.

The main clause is:

• Jeg er i tvivl om = I am unsure about

The clause after that tells you what the uncertainty is:

• hvilken bog jeg skal tage med til universitetet

In standard Danish spelling, subordinate clauses are normally separated by a comma, so the comma here is expected.

Because bog is a common gender noun in Danish: en bog.

The interrogative forms agree with the gender and number of the noun:

• hvilken
  • common gender singular noun
• hvilket
  • neuter singular noun
• hvilke
  • plural

So:

• hvilken bog = which book
• hvilket universitet = which university
• hvilke bøger = which books

You would not say hvad bog here. Hvad usually means what, not which before a noun in this kind of sentence.

Because this is an indirect question inside a subordinate clause, not a direct question.

Compare:

• Direct question: Hvilken bog skal jeg tage med?
• Indirect question: Jeg er i tvivl om, hvilken bog jeg skal tage med.

In Danish subordinate clauses, the subject normally comes before the finite verb:

• jeg skal
• not skal jeg

So the word order is normal subordinate-clause order, even though the clause has question meaning.

It is an indirect question, not a direct one.

A direct question stands on its own:

• Hvilken bog skal jeg tage med til universitetet?

An indirect question is embedded inside another sentence:

• Jeg er i tvivl om, hvilken bog jeg skal tage med til universitetet.

So yes, it expresses a question, but grammatically it behaves like a subordinate clause.

Here skal is best understood as something like should or am supposed to.

The speaker is trying to decide which book is the appropriate one to bring. So:

• hvilken bog jeg skal tage med = which book I should bring / am supposed to bring

It is not a pure future marker here. It has a sense of necessity, suitability, or expectation.

In many contexts, Danish skal can mean:

• must
• have to
• be supposed to
• future with a sense of plan or intention

In this sentence, should / am supposed to is the closest idea.

Tage med means take along or bring.

It is a verb + particle combination:

• tage = take
• med = along / with

Together:

• tage med = take with you, bring along

In Danish, these particles are often separated in clauses:

• Jeg skal tage bogen med.
• Hvilken bog skal jeg tage med?
• ... hvilken bog jeg skal tage med til universitetet.

So med belongs with tage, even though other words can come between them.

A more formal one-word verb would be medbringe, but tage med is very common and natural in everyday Danish.

Because til expresses movement to a place, while på usually expresses being at the place.

So:

• til universitetet = to the university
• på universitetet = at the university / at university

Examples:

• Jeg tager til universitetet nu. = I’m going to the university now.
• Jeg er på universitetet nu. = I’m at the university now.

In your sentence, the idea is bringing the book along when going there, so til universitetet makes sense.

Universitetet is the definite form: the university.

• et universitet = a university
• universitetet = the university

Danish often uses the definite form when the place is understood from context — for example, the university the speaker normally attends.

So the sentence suggests a specific, known university, not just any university.

This is very natural in Danish, even when English might sometimes prefer to university without an article.

Yes. A few natural alternatives are possible, depending on style and nuance:

• Jeg er i tvivl om, hvilken bog jeg skal have med til universitetet.
• Jeg ved ikke, hvilken bog jeg skal tage med til universitetet.
• Jeg er usikker på, hvilken bog jeg skal tage med til universitetet.

Differences:

• i tvivl om = unsure / in doubt
• jeg ved ikke = I don’t know
• usikker på = uncertain about

All are natural, but Jeg er i tvivl om ... sounds especially good when the speaker is weighing options.

Yes, tvivl is a tricky word for many learners.

A rough guide is that it sounds approximately like TVEEL, but that is only approximate. The exact Danish pronunciation is more compressed and has sounds that do not match English perfectly.

A few useful points:

• the t is pronounced
• the v is pronounced
• the word is only one syllable
• the ending -vl cluster can feel awkward for English speakers

You do not pronounce every letter in a slow, fully separate way. It comes out as one tight syllable.

If pronunciation is your goal, it helps to listen to native audio and repeat the full chunk:

• Jeg er i tvivl om ...

Learning the whole expression as one unit is often easier than isolating tvivl by itself.