Dit is de stoel waarop ik altijd zit.
English: This is the chair on which I always sit.
In Dutch, you can start a relative clause with a preposition. This is called 'preposition stranding' in relative clauses. The preposition comes before the relative pronoun (like 'waar', 'wie'), not at the end of the clause.
Use this structure to link two sentences when the preposition is important for the meaning, and you want to be formal or clear. It is common in written Dutch or more formal speech.
Dit is de stoel waarop ik altijd zit.
English: This is the chair on which I always sit.
Het boek waarin ik geïnteresseerd ben, ligt op tafel.
English: The book in which I am interested is on the table.
De collega met wie ik samenwerk, is erg vriendelijk.
English: The colleague with whom I work is very friendly.
De reden waarvoor ik kwam, is belangrijk.
English: The reason for which I came is important.