Follow the same rules for using who(m), which, and that, as you do with other relative pronouns. The only difference is that now these pronouns are functioning as objects:

–The movie that we saw last night was terrible.

–The movie, which we saw last night, was terrible.

For people, you will use either who or whom. Who is usually used instead of whom in colloquial speech, even though it is technically incorrect:

–The person who they saw was sick. (informal)

–The person whom they saw was sick. (formal)

–There's the driver who the police arrested. (informal)

–There's the driver whom the police arrested. (formal)

Examples of Whom, Which and That

• The car which I bought is mid-sized.
• There's the man who they saw last night.