Q: I constantly hear people say “the man that did something” or “the woman that went somewhere.” Shouldn’t it be “the man who did something” or “the woman who went somewhere”? Or did I fall asleep again in English class?
A: Despite what many people believe, a person can be either a “that” or a “who.” There’s no grammatical foundation for the belief that it’s incorrect to refer to a person as a “that” (“the man that I marry,” “the girl that married dear old dad,” and so on).
A thing, on the other hand, is always a “that.” As for pets, they aren’t people, but they aren’t quite things either. If an animal is anonymous, it’s a “that.” If it has a name, it can be either a “that” or a “who.” (“I’m looking for a dog that can act; Lassie is a dog who could direct her own movie.”)
Getting back to people, there may be a “politeness” issue here. Some folks seem to think using “that” in place of “who” or “whom” demeans or objectifies a human being. Still, there’s no grammatical reason for such a rule, even though many style books persist in spreading the misconception.
A Dictionary of Contemporary American Usage, by Bergen Evans and Cornelia Evans, has this to say on the issue: “That has been the standard relative pronoun for about eight hundred years and can be used in speaking of persons, animals, or things. … Three hundred years ago who also became popular as a relative. It was used in speaking of persons and animals but not of things. This left English with more relative pronouns than it has any use for. … Who may in time drive out that as a relative referring to persons, but it has not yet done so.”
For more about these two little words, see the “Who’s That” box in the first chapter of Woe Is I.