Relative clauses begin with a relative pronoun: "der", "die" or "das" in the nominative and "den", "die", das" in the accusative. The number and gender of the relative pronoun is determined by the word it refers to in the main clause. The case of the relative pronoun, however, is determined by its role in the relative clause. It can be the subject or the object in the relative clause.

If the relative pronoun serves as the subject of the subordinate clause, then it is in the nominative case.

Examples:
Es gibt eine Seilbahn. Die Seilbahn fährt bis zum Gipfel.
Es gibt eine Seilbahn, die bis zum Gipfel fährt.

If the relative pronoun serves as an accusative object in the subordinate clause, then it is governed by the accusative case.

Examples:
Er ist ein Mann. Die Fußballfans haben den Mann geschlagen.
Er ist ein Mann, den Fußballfans geschlagen haben.
 

Declension of relative pronouns in the nominative and accusative cases
This chart shows the forms that relative pronouns take in the nominative and in the accusative: