2 A relative clause is a subordinate clause that begins with a relative pronoun. The relative pronoun usually refers back to a noun in the preceding main clause. In English, the words who(m), whose, which and that are used as relative pronouns.
3 In German, there is a single set of relative pronouns corresponding to who(m), whose, which and that. That set of words looks similar to the normal table for the, but with some important differences which are highlighted below.Masculine SingularFeminine SingularNeuter SingularPluralNominativederdiedasAccusativedenGenitivedessenderenDativedemdenenDas Auto dessen Tuer kaputt ist, ist rot The car’s door which is broken is red.
4 Eine erlaubte Droge ist das Koffein, das nicht nur in Kaffee und Tee sondern auch in Schokolade vorkommt.The relative pronoun das is neuter because it refers back to Kaffee and nominative because it is the subject of vorkommt i.e. it is the caffeine which occurs.
5 Sie sind natürliche Stoffe, deren Wirkung man noch abschätzen kann. The relative pronoun deren is plural because it refers back to Stoffe and genitive because it combines with Wirkung to mean the effect of which.