We use who for people, which for things, whose instead of his/her or their, and that for who/which. For example, we would say "This is the boy who played Rugby" using who for a person, "This is the car which had an accident" using which for a thing, and "This is the boy whose mother works at South Florida Bible College" using whose instead of his/her.

1. When do we use the relative pronouns who, which,
whose and that?
who → when we talk about people
which → when we talk about things
whose → instead of his/her or their
We also use that for who/which.
Examples:
This is the boy who played Rugby.
This is the car which had an accident.
This is the boy whose mother works at South Florida
Bible College.