Study the information about relative pronouns.
We should use who and whom for people, and which for things or we can use that for people or things.
We use relative pronouns in the following situations:
1. After a noun, to make it clear which person or thing we are talking about:
the house that Mike built

2. To tell us more about a person or thing:
We had hamburgers, which is my favourite meal.
3. But we do not use that as a subject in this kind of relative clause. We use whose as the possessive form of who:
This is Sam, whose brother went to college with me.
4. We can sometimes use whom as the object of a verb or preposition:
This is Melany, whom you met at our camp last year.
This is Melany’s brother, with whom I went to college.
But now we normally use who.
5. When whom or which have a preposition the preposition can come at the beginning of the clause or at the end of the clause.
We can use that at the beginning of the clause:
Ihad an uncle in Norway that I inherited a bit of money from.