Efter undersøgelsen forklarer lægen, hvilken behandling der passer bedst.

Because Danish main clauses follow the V2 rule: the finite verb usually comes in second position.

Here, Efter undersøgelsen is placed first. That means the verb forklarer must come next, and the subject lægen comes after it:

Efter undersøgelsen forklarer lægen ...

If you started with the subject instead, you would say:

Lægen forklarer efter undersøgelsen ...

Both are correct, but the word order changes because of what comes first.

Undersøgelsen is the definite singular form, meaning the examination.

• en undersøgelse = an examination
• undersøgelsen = the examination

In this sentence, it refers to a specific examination that both speaker and listener know about, so the definite form is natural.

Efter means after and is a preposition.

So:

• efter undersøgelsen = after the examination

It introduces a time phrase telling you when the doctor explains things.

Because that part is a subordinate clause in Danish.

Danish normally uses commas to separate a main clause from a subordinate clause:

Efter undersøgelsen forklarer lægen, hvilken behandling der passer bedst.

English often does not require a comma in the same place, so this can feel unusual to English speakers.

Use hvilken when you mean which and you are choosing among options.

• hvilken = which (singular common gender)
• hvilket = which (singular neuter)
• hvilke = which (plural)

Since behandling is a singular common gender noun, the correct form is hvilken behandling.

Hvad usually means what, not which in this kind of structure.

Because hvilken already functions as the determiner.

In English, we also say which treatment, not which a treatment or which the treatment.

So in Danish:

• hvilken behandling = correct
• hvilken en behandling = wrong
• hvilken behandlingen = wrong in this sentence

Here der is a kind of subject marker in an embedded clause.

The phrase means which treatment suits/is best. The thing being asked about, hvilken behandling, is the subject of passer. In Danish, when a wh-word or wh-phrase is the subject of an embedded question, der is usually inserted:

• Jeg ved, hvem der kommer. = I know who is coming.
• ... hvilken behandling der passer bedst. = ... which treatment fits best.

This der usually is not translated into English.

Passer literally means fits, but in this kind of sentence it often means suits or is the most suitable.

So den behandling der passer bedst means the treatment that fits best, suits best, or is most appropriate.

This is a very natural Danish way to talk about what is most suitable in a situation.

Because bedst is the correct superlative form of god/godt.

Compare:

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

In this sentence, bedst works adverbially with passer:

• passer godt = fits well
• passer bedre = fits better
• passer bedst = fits best

It is an embedded question (also called an indirect question), not a standalone direct question.

Direct question: Hvilken behandling passer bedst? = Which treatment fits best?

Embedded question: Lægen forklarer, hvilken behandling der passer bedst. = The doctor explains which treatment fits best.

So the whole sentence is a statement, but it contains a question-like clause inside it.

Both are in the present tense.

• forklarer = explains / is explaining
• passer = fits / suits

Danish often uses the present tense for general statements, routines, and near-future situations. Here it sounds like a normal factual statement about what happens after the examination.