Today’s two grammar tips:

• 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, but it was happy to receive my other donations. My copy of the book, which my dog ruined, was part of the fourth printing.

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