that, which, who

Of these relative pronouns, that is used to refer to persons, animals, or things; which to animals and things, not persons; and who (whom) to persons only. That is used in restrictive clauses (those that define and limit what precedes by providing information necessary to full understanding): "A man that pays his bills promptly is liked by everyone." Which, who, and whom are used largely in introducing nonrestrictive (not-defining, not-limiting) clauses: "This man, who pays his bills promptly, is liked by everyone." That is often used in illiterate or wordy expressions. For example,"that there" is both wordy and illiterate (that there child)."That is to say" is a wordy way to express "I mean" or "namely." That and which (especially which) are often used in such a way as to create doubt about an antecedent. Avoid saying, for example,"They are coming if their daughter is well enough, which I doubt," because which has no definite antecedent. Say, instead,"... is well enough. However, I doubt that she will be." See also this.