We form Relative Clauses by using relative pronouns and relative pronouns to join two clauses together.
Who is a subject or object pronoun for people.
"Have you met the man who works with me?"
Whose is a possessive for people animals and things.
"Do know anyone whose children go to university?"
Whom is not often used in casual spoken speech. It is used for people, usually in non-defining relative clauses.
"I visited the woman whom I met on vacation."
Which gives us more information about a whole sentence.
"He got an excellent exam result which made me happy."
That can be used to replace whom, who and which.
When is used for a time expression.
"That was the day when we went to the zoo."
Where is used for a place.
"The place where I live is quiet."
Why is used for a reason.
"The great weather is the reason why I moved here."
Defining relative clauses give detailed information about a general term or expression. They are not put in commas.
Non-defining relative clauses give extral information on something, but do not define it. They are not put in commas.
Defining: "Houses which are built of stone are strong." - Houses built of stone are strong. Other types of houses are not as strong.
Non-defining: "Houses, which are built of stone, are strong." - Houses are strong. They are all built of stone.
Defining: "He switched off a TV show that upset me." - I was upset by the TV show.
Non-defining: "He switched off a TV, which upset me." - I was upset because he switched off the TV showI was watching.