Learn Hardcore Danish: Han har ondt i hovedet i aften og vil hvile sig på sengen. - He has a headache tonight and wants to rest on the bed.

Danish often expresses pain with the pattern at have ondt i + body part.
So han har ondt i hovedet literally means he has pain in (the) head.

A few more examples:

Jeg har ondt i ryggen – I have back pain / my back hurts.
Hun har ondt i maven – She has a stomach ache.

This structure is completely standard and idiomatic in Danish.

What is ondt grammatically? Is it an adjective, an adverb, or a noun?

Historically, ondt is the neuter form of the adjective ond (evil, bad), but in this fixed construction have ondt i … it behaves more like a set expression.

You can treat har ondt i as a chunk meaning has pain in.
You do not change ondt for gender or number:

Han har ondt i benet – his leg hurts.
De har ondt i benene – their legs hurt.

Ondt stays the same.

Can I say han er ondt i hovedet instead of han har ondt i hovedet?

No. With pain you almost always use have in Danish, not være:

Jeg har ondt i halsen – I have a sore throat.

Er ondt would be ungrammatical here.
Være is used with adjectives that describe a state, like er træt (is tired), er syg (is ill), but pain is normally expressed with har ondt i ….

What is the difference between han har ondt i hovedet and han har hovedpine?

Both mean that he has a headache, but:

Han har ondt i hovedet – very common, neutral, works for any kind of head pain.
Han har hovedpine – slightly more formal/medical-sounding: he has a headache as a condition (like saying migraine or back pain as a noun).

In everyday speech, har ondt i hovedet is extremely common. Har hovedpine is also correct, just a bit more clinical or precise.

Why is it i hovedet and not på hovedet? In English we say in my head OR on my head.

In Danish, internal pain in a body part is normally i (in), not på (on):

ondt i hovedet – pain inside the head
ondt i ryggen – backache
ondt i maven – stomach ache

På hovedet is used when something is literally on the surface of your head:

Jeg har en hat på hovedet – I have a hat on my head.

So for headache, you want i hovedet.

Could I say han har hovedet ondt or ondt hovedet?

No. That word order is not idiomatic in Danish.

For pain, the standard patterns are:

har ondt i + body part
or a pain noun like har hovedpine, har tandpine

You would not normally put ondt directly before or after the body part in this way.

Why is i aften (tonight / this evening) placed after har ondt i hovedet? Could I move it?

The sentence Han har ondt i hovedet i aften is fine: time expression i aften is at the end of the clause.

You can also move it to the front for emphasis:

I aften har han ondt i hovedet og vil hvile sig på sengen.

Both are correct. Common neutral positions for time adverbials are either at the beginning of the sentence or towards the end of the main clause.

Does i aften mean tonight or this evening? And how is it different from i aftes?

i aften – this evening / tonight (later today, but not yet happened).
i aftes – last night / yesterday evening (already in the past).

So:

Han har ondt i hovedet i aften – he (will) have(s) a headache this evening.
Han havde ondt i hovedet i aftes – he had a headache last night.

Why is vil used in vil hvile sig? Does vil here mean will or wants to?

In this sentence, vil mainly expresses intention: he wants to / is going to rest.

English splits this into will (future) and want to, but Danish vil covers both meanings, depending on context.

To make the meaning would like to / wants to more explicit and polite, Danes often say vil gerne:

Han vil gerne hvile sig – he would like to rest.

What does hvile sig literally mean, and why do you need sig?

At hvile sig is a reflexive verb meaning to rest (yourself).
The reflexive pronoun sig is part of the verb phrase; without it, the meaning changes.

Han vil hvile sig – He wants to rest.
Han vil hvile barnet – He wants to rest the child (make the child rest / lay the child down).

So if the subject is the one resting, you normally use hvile sig.

Can I drop sig and just say han vil hvile på sengen?

You can say han vil hvile på sengen, but then hvile is more like lie / be resting in a general sense, or it may sound slightly more formal or literary.

For everyday he wants to rest (himself), hvile sig is the most natural.
Dropping sig is not wrong, but it changes the tone and sometimes the nuance.

Why is it på sengen and not i sengen?

Both can be correct, but they paint slightly different pictures:

på sengen – literally on the bed, on top of it (maybe not under the covers).
i sengen – in bed, usually under the duvet / using the bed as a place to sleep.

So hvile sig på sengen suggests lying down on the bed, maybe for a short rest.
If he is going to bed properly, many Danes would say hvile sig i sengen or gå i seng (go to bed).

Why is it sengen (the bed) and not just seng (bed)?

Danish normally uses the definite form with specific, known objects:

seng – a bed (any bed, in general).
sengen – the bed (a particular one both speaker and listener know about, usually his own bed).

In a normal home context, sengen is natural because there is a specific bed in mind: his bed / the bed in the room.
Saying hvile sig på en seng would sound like on some bed or other, not the usual situation at home.

Why don’t we repeat han in the second part: …i aften og vil hvile sig på sengen instead of …og han vil hvile sig?

Danish allows you to omit the repeated subject in coordinate clauses when it is the same person and the subject is clear:

Han har ondt i hovedet og vil hvile sig.

You can also say:

Han har ondt i hovedet, og han vil hvile sig.

Both are correct. The version without the second han is more compact and very common in speech and writing.

Is the verb order correct after og? I thought the verb should be in the second position.

Yes, the order is correct. Og links two main clauses:

Han har ondt i hovedet i aften
(Han) vil hvile sig på sengen

In each clause, the finite verb is in the V2 position (second element):

Han (1st) har (2nd) …
Han (1st, implied) vil (2nd) …

Omitting the second han doesn’t change the underlying clause structure; it’s just understood.

How do you pronounce hvile and hovedet? The spelling looks different from what I expect.

Approximate pronunciations (in a rough English-style transcription):

hvile – the h is silent, so it sounds like VEE-le (with a short, unstressed -le).
hovedet – Danish d is soft here, and the final -et is weak; many learners hear something like HO-ðð or HO-ðə.

So the whole phrase might sound roughly like:

Han har ondt i hovedet i aften og vil hvile sig på sengen →
Han har on(t) i HO-ðə i AHF-ten o vil VEE-le sai po SENG-en (very approximate).

Real Danish pronunciation is more reduced and fluid than the spelling suggests.

Is aften common gender or neuter? How do I say an evening and the evening?

Aften is common gender in Danish.

en aften – an evening
aftenen – the evening

With i aften, you use the bare noun (no article) because it is a fixed time expression meaning this evening / tonight, not just any evening.