Breakdown of Slægtsforskning er en hobby på hvilken man bruger meget tid.
være
to be
en
a / an (common)
bruge
to use / to spend (money, time) / to need (together with the modal of obligation)
på
on / at / in (a language) / of (after nouns like end or start)
Questions & Answers about Slægtsforskning er en hobby på hvilken man bruger meget tid.
Why does this sentence use på hvilken instead of the more common som?
Using a preposition followed by hvilken, hvilket, or hvilke is a highly formal, written construction in Danish. It works exactly like saying "on which" in English. In everyday spoken Danish, you would almost never hear this phrasing.
If I wanted to say this in everyday spoken Danish using som, how would the sentence change?
You would use som as the relative pronoun and move the preposition på to the very end of the sentence. The natural spoken version is: Slægtsforskning er en hobby, som man bruger meget tid på.
Why do we use hvilken here and not hvilket?
The relative pronoun must agree with the gender of the noun it replaces. Because it refers back to en hobby (a common-gender noun), we use hvilken. If it referred to a neuter noun like et projekt, you would use på hvilket.
Why is the specific preposition på required here?
In Danish, the fixed phrase for spending time on something is at bruge tid på noget. Because the verb phrase dictates the preposition, på is carried over into the relative clause to link the time spent back to the hobby.