Relative clauses are used to identify, define or classify a thing or a person in a sentence. For this, the relative pronouns "who," which "and" that "will be used. These pronouns are the ones that identify the beginning of the relative clause.
+ Subject relative clause (using the relative pronoun as subject): Noun + subject relative pronoun + verb
I like girls who are intelligent.
(Me gustan las chicas que son inteligentes)
+ Relative clause of object (using relative pronoun as object):
Noun + object relative pronoun + noun / pronoun + verb
I read a book which she recommended me.
(Leí un libro que ella me recomendó)