When to use ‘that,’ ‘which,’ and ‘who’

Who, whom, and whose

Who and whom refer only to people, and whose almost always does so:

“I have a friend who can help.”

“Whom you associate with is your concern.”

“The person whose jacket was left behind is the likely culprit.”

(Whose is sometimes used to refer to an object, as in “Notice the car whose headlights are off.”

This awkward usage should be replaced by, for example, “Notice the car that has its headlights off” or, better, “Notice the car with its headlights off.”)

That

That refers mostly to things, though a class or type of person is also sometimes referred to by this pronoun:

“He has the key that fits in this door.”