Relative pronouns, Relativpronomen, are words that refer to nouns that have been previously mentioned. The most common ones are der, dir, das, welcher, welche and welches. These don’t really translate to English all that precisely, particularly since in German they are declined depending on the grammatical case, but the most common relative pronouns in English are who, which and that.
In German, relative pronouns are used to introduce subordinate clauses in sentences that describe a person or a thing more in depth. This is then a “relative sentence”, Relativsatz, that goes right behind a noun or a pronoun and describe it. (See what we did there? ;) ) You can even chain them one after another so that you describe several attributes in depth.
Main clause noun: der Mann
relative pronoun: der
Das ist der Mann,
This is the man
der einen Porsche gekauft hat.
that bought a Porsche
But how do you determine the relative pronoun?
You need to watch out for only two things:
→ Gender and number must correspond with the noun or pronoun being referenced.
→ The case is dependent on the function that the subordinate clause carries out.
All right, let's try this out.
Das ist der Mann, ich im Supermarkt gesehen habe.
This is the man (that) I saw in the supermarket.
Find the subject. What are we talking about here? ⇒ der Mann.
Define the gender (masculine, feminine, neuter) and number (singular/plural) ⇒ masculine/singular.
What is the subject in the subordinate clause? ⇒ ich. So what does this tell us about the relative pronoun in the subordinate clause and its grammatical case? ⇒ direct object = Accusative ⇒ den.
So we are left with:
Das ist der Mann, den ich im Supermarkt gesehen habe.
Let's have another go:
Das ist der Mann, ich ein Buch gegeben habe.
This is the man (that) I gave a book to.
Main subject? ⇒ der Mann
Gender/number? ⇒ masculine/singular
Subordinate clause: subject? ich; direct ebject? ein Buch; der Mann is therefore the indirect object = Dative ⇒ dem.
So we are left with:
Das ist der Mann, dem ich ein Buch gegeben habe.
Welcher, welche and welches work in exactly the same way.
But: for these relative pronouns there is no genitive form. If you need to use the genitive you have to decline der, die or das.
Welcher, welche and welches are more commonly used in writing rather than in spoken language, particularly when there might otherwise be an article and a pronoun that are the same.
Das ist die Frau, die die erfolgreiche Firma gegründet hat.
⇒ Das ist die Frau, welche die erfolgreiche Firma gegründet hat.
This is the woman that founded the successful firm.
But how do I know which one to use?
Well, for that, there’s no way around it but a little bit of studying and as much practice as you can get! Here’s a table of how to decline the relative pronouns.
Declension of the relative pronouns der, die and das:
der - die - das
dessen - deren - dessen
dem - der - dem
den - die - das
Notice that der, die and das are almost the same as in when they are articles. They change in genitive and in the plural dative form when they end in -en.
Declension of the relative pronouns welcher, welche and welches:
welcher - welche - welches
welchem - welcher - welchem
welchen - welche - welches
That's it! It might seem a bit complicated at first and very, very German, but you'll get the hang of it with some practice ;)