Two Danish relative pronouns

Danish has two relative pronouns, der and som. In English they can be translated as that, which or who. However, they are not always interchangeable. The word der can only be used as the subject of the subordinate clause. 

Here are a few examples:

Jeg har en ven, der elsker øl.

Jeg har en ven, som elsker øl.

The two sentences mean I have a friend who loves beer.

In the next example, only som can be used.

Jeg har en ny ven, som du må møde.

I have a new friend who you must meet.

In the first example, the relative pronoun is the subject of the subordinate clause. For this reason both der and som can be used. In the second example, the relative pronoun  is the object of the subordinate clause. For this reason only som can be used.

The Danish relative pronouns der and som can both be used when they are the subject of the subordinate clause. When the relative pronoun is the object of the subordinate clause, only som can be used. The word som is thus used more extensively than der.