The three sentences that follow are all examples of relative clauses:
Die Frau, die im Wohnzimmer sitzt, ist meine Tante./The woman who is sitting in the living room is my aunt
Der Mann, den ich erkannt habe, hat wahrscheinlich meine Tasche gestolen./The man whom I recognised probably stole my bag
Das Heft, das auf dem Tisch liegt, hat mir meine Mutter geschenkt./My mother gave me the notebook which is on the table
(1) Relative sentences unlike other secondary sentences do not depend on a verb but depend on a noun. As a matter of fact they behave like an extension of that noun. Compare a sentence introduced by dass to a relative clause. In the example given the dass clause depends on the verb 'glaube':
Ich glaube, dass er zu selbstsicher ist.
Die Frau, die im Wohnzimmer sitzt, ist meine Tante.
In grammar, the noun the relative clause depends on (in the example Die Frau) is called antecedent of the relative sentence.
(2) Relative sentences are introduced by the relative pronouns 'der, die, das'. Relative pronouns pattern almost exactly like articles (there are a couple of exceptions we will discuss once you are more familiar with how these sentences work). Relative pronouns send the verb to the end of the sentence.
(3) Relative clauses need to come immediately after the name they refer to (again, an exception to this rule will be discussed at a later point).
(3) Relative clauses need to come immediately after the name they refer to (again, an exception to this rule will be discussed at a later point).
(4) The exact form of the relative pronoun at the beginning of the relative clause depends on two things: (a) the gender and number of the antecedent and (b) the function of the pronoun in the relative sentence.
The gender and the number of the pronoun are determined by the antecedent
The case of the pronoun is determined by the function it has in the relative clause
Now, have the table with the full pattern of relative pronouns in front of you (p. in the grammar) and let's go through an example:
Der Mann, den ich kenne, ist blond./The man (who) I know is blonde
How do you work out that the pronoun you need to introduce the relative clause here is 'den'?
You ask yourself two questions:
(1) What gender and number is the antecedent (Der Mann/the man)? Answer: It is masculine and singular (at this point you know that you need 'der' in some of its forms)
(2) What function does the pronoun have in the relative sentence: is it the subject, a direct object or does it depend on a preposition? Answer: In this case the pronoun is the direct object because 'I know the man/him', hence the pronoun goes in the accusative, which is the case of direct objects.
Summarising, our pronoun needs to be masculine, singular and accusative = den
