Relative clauses are used extensively in German, and they use dependent word order (i.e. the conjugated verb moves to the end of the clause). In addition, the grammar of the relative clause remains independent of the grammar of the main clause.
Relative clauses in English use the relative pronouns that, which, who, whom, and whose. In German, the relative pronouns are nearly identical to the definite articles, with the exception of denen in the dative plural (instead of den) and dessen and deren in the genitive (instead of des and der).
For example:
Das ist das Haus, das ich gekauft habe.
That is the house that I bought.
In the above example, “das Haus” is a predicate noun (it is a renaming of the subject, “that”; “that” = “the house”), so it is in the nominative case. In the relative clause, the house (represented by “that”) is the direct object: “I” is the subject performing the action (“bought”), and the house is the thing being bought (i.e. recipient of the action, or direct object). This example shows how the function of the noun or “antecedent” in the main clause does not necessarily stay the same in the relative clause.
Relative clauses can follow other relative and subordinate clauses, thereby creating lengthy or nested sentences (Schachtelsätze; literally: “box” sentences, which are not unlike the concept of nesting dolls). Consider the following example:
Ich habe einen Mann gesehen, der sich verhalten hat, als ob er Superman wäre, der dafür bekannt ist, dass er fliegen kann.
I saw a man who behaved as if he were Superman, who who is known for the fact that he can fly.
The boldface words are relative pronouns (der) and subordinating conjunctions (als ob, dass), which allow for the construction of more complex sentences (Schachtelsätze). These sentences are more frequently seen in print and less often heard in conversation, due to their length and difficulty in following. Although these are present in written German, particularly in technical, academic, and literary texts, they are not prevalent in the vernacular or everyday speech (Alltagssprache).
German Relative clauses allow us to provide additional information about a noun without starting a new sentence. Relative clauses are usually introduced by relative pronouns, mostly der, die, das (for both people and things), and not by wer/wen, even though English uses who/whom for people. (Was, wo and wer can also introduce relative clauses in certain cases, e.g. Das ist alles, was ich weiß.) We can use relative pronouns to combine two main clauses. In German grammar, relative clauses are always set off by commas.