Learn Hardcore Danish: Jeg spørger min ven, hvornår han kommer til huset. - I ask my friend when he is coming to the house.

Jeg spørger min ven, hvornår han kommer til huset.
Literally: I ask my friend when he comes to the house.
Natural English: I ask my friend when he is coming to the house / when he will come to the house.

You only need a clear future form (like vil komme) when you really want to stress intention, possibility, or uncertainty. In most everyday contexts, simple present kommer is enough to express future time, especially when there is a time reference (here, hvornår “when”).

Why is it hvornår han kommer til huset and not hvornår kommer han til huset?

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

Direct question:
Hvornår kommer han til huset?
When is he coming to the house?
→ Verb comes right after hvornår.
Indirect question (embedded in a larger sentence):
Jeg spørger min ven, hvornår han kommer til huset.
I ask my friend when he is coming to the house.
→ In Danish subordinate clauses, the subject comes before the verb:
- han kommer, not kommer han.

So:

Question on its own → Hvornår kommer han … ?
Question inside a sentence → hvornår han kommer …

What is the difference between hvornår and når?

Both relate to “when”, but they are used differently:

hvornår = interrogative “when?”, used in (direct or indirect) questions:
- Hvornår kommer han til huset? – When is he coming to the house?
- Jeg spørger, hvornår han kommer til huset. – I ask when he is coming to the house.
når = “when(ever)” in statements, not in questions:
- Når han kommer til huset, ringer han til mig.
  When he comes to the house, he calls me.

You cannot use når to introduce a question. That’s why in this sentence it must be hvornår.

Why is there a comma before hvornår?

The comma marks the beginning of a subordinate clause (hvornår han kommer til huset).

Jeg spørger min ven, ← main clause
hvornår han kommer til huset. ← subordinate (indirect question)

Traditional Danish grammar requires a comma before such clauses. Modern spelling rules make some commas optional, but you will very often see (and be taught) the comma before hvornår, at, fordi, når, hvis, etc.

So the comma is there because a new clause, introduced by hvornår, begins.

Why is it min ven and not mit ven or mine ven?

The possessive “my” changes depending on the gender/number of the noun:

min – for en-words in singular
- en ven → min ven (my friend)
- en bil → min bil (my car)
mit – for et-words in singular
- et hus → mit hus (my house)
- et barn → mit barn (my child)
mine – for all plurals (both en-/et-words)
- venner → mine venner (my friends)
- huse → mine huse (my houses)

Since ven is an en-word (en ven), it takes min: min ven.

Why is it han and not ham in hvornår han kommer til huset?

Han and ham are two forms of “he”:

han = subject form (like English he)
ham = object form (like English him)

In the clause hvornår han kommer til huset, the person (he) is the subject of the verb kommer:

han (subject) + kommer (verb) → correct

Ham would be used as an object:

Jeg ser ham. – I see him.
Jeg venter på ham. – I wait for him.

So you say:

hvornår han kommer, not hvornår ham kommer.

Why do we say til huset and not i huset or på huset?

The preposition and form depend on the meaning:

til huset – literally “to the house”
→ Focus on movement towards the house (destination).
- Han kommer til huset. – He is coming to the house.
i huset – “in the house”
→ Being inside the house (location).
- Han er i huset. – He is in the house.
på huset – literally “on the house”
→ Physical “on top of” the house, or idiomatic in some fixed phrases, but not normal for simple “to the house.”

In your sentence, the important idea is going to that place, so til huset is the natural choice.

Why is it huset and not just hus?

Danish usually marks definiteness with an ending on the noun:

hus – house (indefinite)
huset – the house (definite)

The English meaning here is “the house” (a specific house, probably known to both speakers), so Danish must use the definite form:

til huset = “to the house” (a particular house)
til et hus = “to a house” (to some house or other; unusual here)

So huset is required to match the definite meaning.

Could you also say Jeg spørger min ven, hvornår han vil komme til huset? What is the difference?

You can say it, but it is less natural in everyday speech.

Jeg spørger min ven, hvornår han kommer til huset.
→ Neutral, normal way to talk about when he is coming (future).
Jeg spørger min ven, hvornår han vil komme til huset.
Literally: … when he will come to the house.
→ Sounds more like you are focusing on his intention or willingness:
- when he is willing to come
- when he is going to decide to come

So:

For a simple factual “when will he arrive?”, use kommer.
Use vil komme when you really want to stress intention/decision or a more distant/uncertain future.

Why is it Jeg spørger min ven and not something like Jeg spørger til min ven or Jeg spørger min ven om?

The verb spørge (“to ask”) works like this:

spørge + person + om + noget
- Jeg spørger min ven om tiden. – I ask my friend about the time.
spørge + person + hv- clause (indirect question – no om)
- Jeg spørger min ven, hvornår han kommer til huset.
  I ask my friend when he is coming to the house.

So in your sentence:

min ven is the person you ask,
the hvornår… clause is the thing you ask about,
therefore you do not add om.

Spørge til exists but means something different (ask/inquire about someone’s situation):

Lægen spørger til patientens helbred. – The doctor asks about the patient’s health.

That is not the meaning here.

How would this sentence change if I turned it into a direct question to my friend?

The current sentence reports what you do:

Jeg spørger min ven, hvornår han kommer til huset.
I ask my friend when he is coming to the house.

If you speak directly to your friend, you would say:

Hvornår kommer du til huset? – When are you coming to the house?

Changes:

The reporting part Jeg spørger min ven disappears.
The pronoun changes from han (he) to du (you).
Word order becomes that of a direct question: Hvornår kommer du … ? instead of hvornår du kommer ….