Jeg er i tvivl om, hvilken skjorte der passer bedst til mødet i morgen.

At være i tvivl om is a very common Danish expression meaning to be unsure about or to be in doubt about.

• Jeg er i tvivl om ... = I’m unsure about ...
• Jeg tvivler på ... more often means I doubt ... in the sense of not really believing something.

So in this sentence, jeg er i tvivl om is the natural way to say that the speaker is uncertain and trying to decide something.

Here, om means something like about.

So:

• i tvivl om noget = in doubt about something
• Jeg er i tvivl om, hvilken skjorte ... = I’m unsure about which shirt ...

This is just the normal preposition that goes with the expression at være i tvivl om.

Hvilken means which and is used with a singular common-gender noun.

• en skjorte = a shirt
• so: hvilken skjorte = which shirt

Compare:

• hvilken for singular common gender: hvilken skjorte
• hvilket for singular neuter: hvilket møde
• hvilke for plural: hvilke skjorter

Hvad does not work here because Danish normally uses hvilken/hvilket/hvilke for which + noun.

In Danish, a comma is commonly used before a subordinate clause or embedded clause.

Here the sentence has:

• main clause: Jeg er i tvivl om
• embedded question: hvilken skjorte der passer bedst til mødet i morgen

So the comma marks the beginning of that embedded clause.

Because this is an embedded question, not a direct question.

Direct question:

• Hvilken skjorte passer bedst til mødet i morgen?

Embedded question:

• Jeg er i tvivl om, hvilken skjorte der passer bedst til mødet i morgen.

In embedded questions, Danish does not use normal question word order. The clause becomes more like a statement in structure.

Here der acts as the subject of the embedded clause.

You can think of it as meaning:

• which shirt is the one that fits best

The structure is very common after hvilken/hvilket/hvilke when that word refers to the subject of the following clause.

So:

• Hvilken skjorte passer bedst? = direct question
• ... hvilken skjorte der passer bedst = embedded version

This der is not translated directly in natural English, but it is part of normal Danish grammar.

The verb at passe has several meanings, and one common meaning is to suit or to be suitable for.

So:

• skjorten passer til mødet = the shirt suits the meeting
• passer bedst til mødet = suits the meeting best

The preposition til is the usual one here when talking about what something is suitable for.

Bedst is the superlative form of godt when used adverbially.

The forms are:

• god = good
• bedre = better
• bedst = best

Since bedst modifies passer, it works like best in English:

• passer bedst = fits/suits best

This is just an irregular comparison, like good → better → best in English.

Mødet is the definite form, meaning the meeting.

• et møde = a meeting
• mødet = the meeting

Since the sentence refers to a specific meeting, namely the one tomorrow, Danish uses the definite form:

• til mødet i morgen = for the meeting tomorrow

In Danish, i morgen is the normal fixed expression for tomorrow.

So you simply say:

• i morgen = tomorrow
• mødet i morgen = the meeting tomorrow

This is just standard Danish usage.

Skjorte is a common-gender noun, which is why the sentence uses hvilken.

You can tell because:

• the basic noun is en skjorte
• common-gender singular nouns take hvilken

If it were a neuter noun, you would use hvilket instead.

Yes, hvad for en skjorte is possible in spoken Danish and often sounds a bit more conversational.

Compare:

• hvilken skjorte = more standard, neat, and common in writing
• hvad for en skjorte = more informal/spoken, roughly which shirt

In this sentence, hvilken skjorte is the most natural written choice.

Yes. The main verb er is present tense because it describes the speaker’s current state:

• Jeg er i tvivl ... = I am unsure ...

The fact that the meeting is in the future is shown by i morgen, not by changing the tense of er.

That works much like English:

• I’m not sure which shirt is best for tomorrow’s meeting.