Relative pronouns
Incorrect : I know the man which came here.
Correct : I know the man who/that came here.
Incorrect: I killed the rat who disturbed me.
Correct : I killed the rat which/that disturbed me.
Note: The relative pronoun ‘who’ refers to persons only. However ‘that’ can refer to all kinds of antecedents - both persons and things/animals. Remember, the same relative pronoun can refer to both singular and plural antecedents.
For example:
I know the girl/girls who/that came here.
I have lost the book/books which /that you gave me.