A relative clause is a way of further describing something or somebody. In English they are introduced using: 'who', 'that', 'which' or 'what'.
eg: The man who lives opposite me is ugly
In German they are introduced by a relative pronoun which is governed by case and gender, as the following table shows:
| masc | fem | neut | plur | |
| nom | der | die | das | die |
| acc | den | die | das | die |
| gen | dessen | deren | dessen | deren |
| dat | dem | der | dem | denen |
We need to decide on number and gender, and on case.
The number and gender is found by looking at the thing or person that the relative clause is describing.
eg1: The man who lives opposite me is ugly.
In this example we are talking about a man (ein Mann), so he is singular and masculine.
To find the case we need to look at the grammatical role that the pronoun is playing in the relative clause. In this instance the man is the subject of the verb 'to live'. Therefore, he will be in the nominative case.(der)
Der Mann, der mir gegenüber wohnt, ist häßlich
eg2: The jacket that I bought only cost €20
Gender and number: sing. fem. (die/eine Jacke)
Case: Accusative. The jacket is not doing the buying, it is being bought so it is a direct object. Thus the relative pronoun is die
Die Jacke, die ich gekauft habe, hat nur €20 gekostet
Relative clauses function a little like subordinate clauses in that:
If we need to use a preposition, we must put this DIRECTLY BEFORE the relative pronoun. The case of the pronoun is then directly governed by the preposition itself.
eg: the town that I live in is called Cheltenham.
The preposition here is 'in' which must go straight before the relative pronoun and put it in the dative: - 'die Stadt, in der ich wohne, heißt Cheltenham
eg2: the girl I'm doing my homework with is really nice: das Mädchen, mit dem ich meine Hausaufgaben mache, ist wirklich nett.
This is used whenever we would say 'of whom' or more commonly 'whose' in English.
eg: The man whose daughter is ill is very sad: der Mann, dessen Tochter krank ist, ist sehr traurig.
The children whose work I am marking are very lazy: die Kinder, deren Aufgaben ich korrigiere, sind sehr faul.