Both som and der introduce a relative clause, but der can only stand for the SUBJECT of that clause. Som works for the subject or the object, and when the pronoun is the object it can be left out entirely. In everyday Danish der is the usual choice for a subject.
Examples
- Bilen, der holder derovre, er min.
- The car that is parked over there is mine.
- Bilen, som jeg købte i går, er rød.
- The car that I bought yesterday is red.
- Bilen, jeg købte i går, er rød.
- The car I bought yesterday is red.
- Vindmøller, der står på havet, larmer mindre.
- Wind turbines that stand at sea make less noise.
The mistake to avoid
Using der for an object ("bilen, der jeg købte") - der is subject-only; use som or drop the pronoun.
Where this fits in
Miljø og bæredygtighed, Danish B2: Talk about climate, consumption and transport, and debate who is responsible for the green transition.