Relative clauses add information about a noun in the main clause. In Dutch, the relative pronoun is either die or dat, chosen based on the noun’s article. Die is used with de-nouns and all plurals: “de man die daar staat” (the man who stands there), “de boeken die ik heb gelezen” (the books that I have read). Dat is used with het-nouns in the singular: “het huis dat ik wil kopen” (the house that I want to buy).
The relative clause is a subordinate clause — the verb goes to the end: “de vrouw die elke dag hardloopt” (the woman who runs every day). When a preposition is involved, Dutch uses waar- + preposition instead of a pronoun + preposition: “het boek waarover ik sprak” (the book about which I spoke — literally “the book whereof I spoke”). This waar- compound is essential and has no direct English parallel.
For people combined with a preposition, you can use die/dat + preposition or the waar- form: “de vriend met wie ik ging” or informally just “de vriend die ik mee ging.” The formal form uses wie after prepositions for persons. In informal speech, Dutch speakers often simplify these constructions. Learning the formal written versions first, then recognizing the informal spoken variants, is the recommended learning path.