The relative pronouns that, which, and who are frequently interchanged. This article provides grammar rules for using these pronouns correctly.
1. Grammar Rules: That, Which, and Who– Choose the pronoun that to introduce a restrictive relative clause.
Oxford Dictionaries Online defines a restrictive relative clause as a clause that “… gives essential information about a noun that comes before it.”
Consider this example:
The cutlet knife that is on the counter needs to be sharpened.
In that example, the restrictive relative clause introduced by that gives essential information about the noun, cutlet knife; that is to say, the clause implies that there is more than one cutlet knife and identifies the specific cutlet knife that needs to be sharpened.
2. Grammar Rules: That, Which, and Who – Choose the pronoun which to introduce a nonrestrictive relative clause.
Oxford Dictionaries Online defines a nonrestrictive relative clause as a clause that “… provides extra information that could be left out without affecting the meaning or structure of the sentence.” Stated differently, a nonrestrictive relative clause gives nonessential information about the noun that comes before it.
Consider this variation of the above example:
The cutlet knife, which is on the counter, needs to be sharpened.
In that example, the nonrestrictive relative clause introduced by which does not give essential information about the noun, cutlet knife. Unlike the first example, this clause implies that there is just one cutlet knife; therefore, the clause can be removed without changing the meaning of the sentence.
3. Grammar Rules: That, Which, and Who– Choose the pronoun who (and its related forms) when referring to people.
According to Rutgers University professor Jack Lynch, Ph.D., “You should usually use who (and its related forms, whose and whom) only to refer to people, with that or which only for non-human things ….” Grammarian Mignon Fogarty agrees: “To me, using that when you are talking about a person makes them seem less than human. … [Y]ou wouldn’t want to do that accidentally.” As Fogarty points out, however, the American Heritage Dictionary takes the position that either who or that is permissible.
Consider these two examples:
Julie is the one who sent the gift to me.
Julie is the one that sent the gift to me.
I side with the linguistic prescriptionists: that is dehumanizing.
Fogarty, Mignon. “Who versus That,” Grammar Girl
Lynch, Jack Ph.D. “Who versus That or Which,” Guide to Grammar and Style
“‘That’ or ‘Which’,” Oxford Dictionaries Online
“Which vs. That,” The Chicago Manual of Style Online