“He has some friends which I know.” Whom, the objective case form of the pronoun who, should here be used.
“The dog, who was called Rover, went mad.” Use which.
What, That
That is applied to persons, animals, and things. What is applied to things. The antecedent of what should not be expressed. What is both antecedent and relative.
“All what he saw he described.” Say, “What he saw,” or “All that he saw,” etc.
Uniform Relatives
When several relative clauses relate to the same antecedent, they should have the same relative pronoun.
“It was Joseph that was sold into Egypt, who became governor of the land, and which saved his father and brothers from famine.” Change that and which to who.
Choice of Relatives
Since who and that are both applied to persons, and which and that are both applied to animals and things, it often becomes a serious question which relative we shall employ. Much has been written upon the subject, but the critics still differ in theory and in practice. The following is probably as simple a statement of the general rule as can be found: