Quite often there is a preposition before the relative pronoun.

Have a look at these phrases:   The church, in front of which you are, was built in 1456.   The law, against which we had fought, was finally abandoned.   The house, in which the accident happened, was sold.   This was it exactly, against which we were fighting.

It's the same in German, just that we have the challenge of the declination, because every preposition requires a different declination.

Examples     Die Schachtel, in der er das Geld versteckt hatte, fiel vom Tisch. = The box, in which he had hidden the money, fell from the table.   Die Schachtel, in die er das Geld gelegt hatte, fiel vom Tisch. = The box, into which he had put the money, fell from the table.   Die Schachtel, auf der er gesessen hatte, fiel vom Tisch. = The box, on which he had sat, fell from the table.   Die Schachtel, mit der er gespielt hatte, fiel vom Tisch. = The box, with which he had played, fell from the table.

The fact that each preposition requires a certain declination we have already explained in the chapter about more prepositions. This, of course, doesn't make things easier, but basically nothing really changes. The relative pronoun is still determined by the noun that it refers to and the declination is determined by the preposition of the relative clause.

Let's have a look at an example We have two phrases   Der Mann ist ein Lehrer. = The man is a teacher.   Ich habe mit ihm gesprochen. = I have talked to him.   These two we combine to one: The man, to whom I talked, is teacher.

The logic is the same like the ones before. The main phrase is Der Mann ist ein Lehrer. The object to which the relative phrase refers is der Mann. Der Mann is singular masculine. In the main phrase der Mann is in Nominative, but this is not the interesting part. The interesting part is, that der Mann is in Dative-declination in the relative phrase, because the preposition mit requires the Dative.

The relative pronoun that we need is in masculine singular with Dative-declination. According to the table it is dem. The phrase is:

Der Mann, mit dem ich gesprochen habe, ist Lehrer.