Where, referring to a place, why, referring to a reason, and when, referring to a time, can be used instead of a relative pronoun after a noun.
In defining relative clauses why and when, unlike where can be omitted.
Example: I'd like to know the reason (why) he decided not to come.
February is the month (when) many of my colleagues take skiing holidays.
BUT! She always had wanted to go to a place where she could speak her native tongue.
When, where and why are not omitted in non-defining relative clauses.
Example: I come from the Seattle area, where many successful companies such as Microsoft and Boeing are located, and I often go home during the summer.
He likes shopping between one and three, when most people are at home, because of the relative calm.
When speaking, we often omit the relative pronoun.
Whom is formal and most often used when writing.
Relative clauses and prepositions
In formal English prepositions can come before the relative pronoun. However, it much more common to place prepositions at the end of the relative clause, especially in informal spoken English.
Example: John Robbins, whom I spoke to by telephone, instructed me to buy 200 shares of WAKO. Formal
The Ritz, which was stayed at in New York, was extremely expensive.
Defining Relative Clauses
Example: The banker to whom I gave my check was quite friendly. - formal
The woman I talked to was very pleasant indeed. - informal
The book which I received for my birthday was excellent. - formal
The car he drove was really fast. - informal
Example: The bank manager, to whom he addressed his complaints, was very unhelpful. - formal.
The local branch manager, who I talked to about my problems, was very helpful. - informal