Gramma
relative pronoun: subject case
Use who or that when th antecedent is a person, and use which or that when the antecedent is an animal or object.
ex: that’s the boy who came to our party last week
Note: that, as a relative pronoun, only laeds an indentifying clause, namely, the relative clause cannot be surrounded by commas.
B. Use what . When it is not preceded by a noun, or an “antecedent”
Ex: please tell us what you have in mind
C. Which can refer to the entire preceding clause
Note: when it refer to the preceding clause. Which disapponinted his father
note: when it refers to the preceding clause, which cannot be replaced either by that or by what