Hun er i gang med at slå et ord op, fordi hun er i tvivl om, hvilken modsætning der passer bedst.

It is a very common Danish expression meaning to be in the middle of doing something or to be busy doing something.

Structure:

• være i gang med at + infinitive

So:

• Hun er i gang med at slå et ord op = She is in the middle of looking up a word.

You can also use i gang med before a noun:

• Jeg er i gang med lektierne = I’m working on the homework.

Because slå op is a fixed verb expression, often called a particle verb, meaning to look up.

In Danish, these are often split when there is an object:

• slå et ord op = look up a word
• slå det op = look it up

So op belongs with slå, but it appears after the object.

Compare:

• Hun slår ordet op
• Hun vil slå ordet op
• Hun er i gang med at slå ordet op

This is completely normal in Danish.

Et ord means a word, while ordet means the word.

Here, et ord is indefinite, so the sentence presents it as just some word she is looking up, not a specific word already identified in the sentence.

• et ord = a word
• ordet = the word

If the word had already been mentioned, ordet might be more natural.

Yes, in this context it means to look up a word, usually in a dictionary, word list, or similar reference source.

You can also make the source explicit:

• slå et ord op i en ordbog = look up a word in a dictionary

Danish often leaves the dictionary unstated if it is obvious from context.

Være i tvivl om is an idiomatic expression meaning to be unsure about, to be uncertain about, or to be in doubt about.

So:

• hun er i tvivl om ... = she is unsure about ...

Examples:

• Jeg er i tvivl om svaret = I’m unsure about the answer
• Vi er i tvivl om, hvad vi skal gøre = We’re unsure what we should do

The preposition om is part of the normal expression here.

Because modsætning is a common gender noun in Danish:

• en modsætning

So the correct question word is:

• hvilken for en-words
• hvilket for et-words
• hvilke for plural

That gives:

• hvilken modsætning = which opposite / which antonym

Hvad would not fit as well here, because hvilken is used when choosing among specific alternatives.

Here modsætning most likely means opposite or antonym.

So:

• hvilken modsætning der passer bedst = which opposite/antonym fits best

In other contexts, modsætning can also mean:

• contrast
• opposition
• contradiction

But in a sentence about looking up a word, the most natural meaning is antonym.

In hvilken modsætning der passer bedst, the word der introduces a clause where it functions as the subject.

Very literally:

• which opposite that fits best

In natural English, we do not translate der here directly. We just say:

• which opposite fits best

This use of der is very common in Danish when the relative word is the subject of the clause.

Compare:

• manden, der kommer = the man who is coming
• bogen, der ligger på bordet = the book that is lying on the table

Because passer bedst means fits best or is the most suitable in this context.

That is slightly different from er bedst, which can sound more general, like is the best in an absolute sense.

For word choice, Danish often prefers passe:

• Det ord passer bedst = That word fits best
• Hvilken modsætning passer bedst? = Which antonym fits best?

So passer bedst is very natural here because she is choosing the most appropriate word.

Because fordi introduces a subordinate clause, and subordinate clauses in Danish do not use normal main-clause word order.

Main clause:

• Hun er i tvivl

After fordi:

• fordi hun er i tvivl

So the pattern is:

• conjunction + subject + verb ...

This is one of the most important word-order differences for English speakers learning Danish.

Because after i gang med plus a verb, Danish normally uses at + infinitive:

• i gang med at læse
• i gang med at skrive
• i gang med at slå et ord op

So:

• Hun er i gang med at slå et ord op

If i gang med is followed by a noun instead of a verb, there is no at:

• Hun er i gang med arbejdet = She is busy with the work

It is the most natural choice here, yes.

Fordi is the standard word for because when giving a reason:

• Hun slår et ord op, fordi hun er i tvivl = She looks up a word because she is unsure

Danish also has for, which can sometimes mean for/because, but it works differently and is often more like an explanation added afterward.

In this sentence, fordi is clearly the best and most normal option.

Yes. Danish often allows small variations without changing the basic meaning.

For example:

• Hun er ved at slå et ord op, fordi hun er i tvivl om, hvilken modsætning der passer bedst.
• Hun slår et ord op, fordi hun er i tvivl om, hvilken modsætning der passer bedst.

Differences:

• er i gang med at = emphasizes that the action is in progress
• er ved at = also means she is in the process of doing it
• slår et ord op = more neutral/simple present

The original sentence is completely natural and emphasizes the ongoing action very well.