More Than Just der, die, and das
You recognize der, die, and das as definitive articles. But these words also have another use in German. They are also relative pronouns.
A relative pronoun refers to someone or something already mentioned in a sentence. In English we use who, that, and which.
Where's the man who needs a doctor?They finally found the car that was stolen.The book that he chose for his report is too short.
German forms the same kind of constructions, which are called relative clauses, but uses the definite articles in place of the words who, that, and which. And, of course, gender plays a key role. If the person or thing you're talking about is masculine, you have to use der as the relative pronoun. If it's feminine or plural, use die. If it's neuter, use das. Then, the conjugated verb has to stand at the end of the clause.
Let's look at some examples.
Ich kenne den Mann, der an der Ecke steht.I know the man who's standing on the corner.Seine Freundin, die aus Schweden kommt, will Lehrerin werden.His girlfriend, who comes from Sweden, wants to become a teacher.Ich sehe die Kinder, die ins Kino gehen.I see the children who are going to the movies.Sie kaufen ein Haus, das sehr alt ist.They are buying a house that is very old.
This usage of der, die, and das as relative pronouns is easy to identify. Look for two things: (1) The gender of the article is the same as the noun to which it refers, and (2) the conjugated verb in the clause is at the end of the sentence. Er findet einen Hund, (1)
Of course, if the relative pronoun is used as a subject, it will be in the nominative case as shown in the previous examples. But relative pronouns can appear in the other cases—dative, accusative, or genitive—as well. Although the relative pronouns are different from definite articles, they still change with the functions of the cases just like definite articles.
Accusative Direct Object: Wo ist der Student, den Sabine liebt?Where is the student that Sabine loves?Dative Preposition: Wo ist der Diplomat, mit dem Karl gesprochen hat?Where is the diplomat that Karl spoke with?