Pronouns that introduce relative clauses and refer back to a noun or pronoun mentioned earlier.
In German, a relative pronoun takes its gender and number from the noun it refers back to, and its case from its role in the relative clause.
Forms
They look like the definite article, except in the dative plural and the genitive. In each line: masculine, feminine, neuter, plural.
Nominative: der, die, das, die
Accusative: den, die, das, die
Dative: dem, der, dem, denen
Genitive: dessen, deren, dessen, deren
Examples
Das ist der Mann, der hier wohnt.
That is the man who lives here.
Das ist der Mann, den ich gestern gesehen habe.
That is the man I saw yesterday.
In both, Mann is masculine, so the pronoun is masculine. In the first, the man is the subject of the clause, so der is nominative. In the second he is the object of gesehen habe, so den is accusative.
See also
Pronoun
Personal pronoun
Possessive pronoun
Reflexive pronoun