- Use the word “who,” not “that,” when referring to a person. (“Anna is the swimmer who had the best time,” not “Anna is the swimmer that had the best time.”) “That” and “which” refer to groups or things. (“Anna is on the team that won the gold medal.”)
- “That” introduces essential clauses, and “which” introduces nonessential clauses (those adding information that is nice but not necessary). The library refused to accept the book that my dog ruined. The book, which my dog ruined, made its debut more than 20 years ago.
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