A. Who, which, that
Who is used when the antecedent is a person, while which is used when the antecedent is not a person. However, that as a relative pronoun can replace either who or which, and it is preferred especially after all, everything, nothing, the only..., and superlatives.
- The woman is a singer. + She lives next door. (= The woman who / that lives next door is a singer.)

B. Notice that which can refer to the entire preceding clause.
- John didn't pass the test, which (NOT that or what) disappointed his father.

C. Possessive of the relative pronouns
Possession can be expressed by whose, whether or not the antecedent is a person. When the antecedent is not a person, of which - . . may also be used.
- The man is my uncle. + His car broke down. (= The man whose car broke down is my uncle.)

D. What (= the thing which)
The relative pronoun what is different from the other relative pronouns in that it is not preceded by a noun.
- He believes what he hears. (= He believes the thing that/which he hears.)