The correct use of the relative pronouns “who”, “that”, and “which” relates the subject of a sentence to its object. The question of which of the three words to use in a given context confuses some writers, so here’s an explanation of their appropriate roles:
Who, Whom, and Whose
“Who”, “whom”, and “whose” refer only to people.
Example: I have a potnah who can help.
Example: Whom you associate with is your business.
Example: The girl whose jacket was left behind most likely did it.
Please note: “Whose” is sometimes used to refer to an object as in “Notice the car whose headlights are off,” but this awkward usage should be replaced with “Notice the car with its headlights off.”
“That” refers mostly to things; although, a class or type of person is also sometimes referred to by this pronoun.
Example: He has the key that fits in this door.
Example: This is a squad that is going places.
Example: He’s the kind of doctor that volunteers on his day off.
Please note: Even though the previous sentence is technically correct, it’s usually best to maintain a distinction between ‘people’ and ‘not-people’ by using “who” in reference to a type of person:
Example: He’s the kind of doctor who volunteers on his day off.
The use of “that” in association with people itself is attested, but a class of people is always considered a thing, not a person.
“Which”, like “that”, refers to things, but it is frowned upon when it appears in a restrictive or essential clause.
Example: I chose the card which is blank.
This sentence, which specifies a card among one or more others that are not blank, has a meaning distinct from “I chose the card, which is blank,” which refers to a single card and then describes it. (This is an example of a nonrestrictive or nonessential clause. This form is sometimes called ‘nonessential’ because the information that follows “which” is not required.)
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