Relative clauses give extra information without making you start a new sentence.
They are subordinate clauses (verb at the end!) and have a relative pronouns at the beginning. They are separated from the rest of the sentence by commas.
Example: „Das ist der Mann, der einen Ferrari hat.“
If we want to give additional information about a person or a thing, we use the relative pronouns "der," "die," "das," or "welche."
They have to be declined. But how?!?!?
Which noun from the main clause does the relative clause describe?
Answer: "der Mann"
What gender and number does that noun ("der Mann") have?
Answer: masculine, singular
What case does the relative pronoun have IN THE SUBORDINATE CLAUSE?
Pronoun = masculine + singular + nominative ⇒ "der"
You can learn more about how to use relative pronouns in different situations in this lesson: Relative Pronouns.
If the relative clause describes a place, we often use the relative pronoun "wo" at the beginning of the relative clause.
The "wo" replaces the relative pronoun + preposition.
In informal language, "wo" is almost always used.
Relative clauses can also describe demonstrative pronouns and indefinite pronouns.
When it is a neuter superlative, something non-specified ("etwas," "alles," "nichts," …), or an entire clause, we use "was."
In this case it isn't possible to include a preposition. However, most of the time the relative clause that relates to an entire sentence is constructed with wo(r)+ preposition.
In informal speech we would always just say: