By Judy Vorfeld
Let’s take this sentence: “The runner who exercises regularly usually does the best.”
“Who” connects the subject, runner, to the verb “exercises.”
Many people will say “The runner that exercises usually does the best.”
Here’s the thing: “who” (and its forms) refers to people. “That” usually refers to things, but it can refer to people in a general sense (like a class or type of person: see “runner.”). Purdue Online Writing Lab says, “When referring to people, both that and who can be used in informal language. ‘That’ may be used to refer to the characteristics or abilities of an individual or a group of people.… However, when speaking about a particular person in formal language, who is preferred.”
That said, many people and some respected references prefer “people that,” and it’s not wrong. Think Chaucer. Shakespeare. Dickens.
Bottom line: be consistent.