The Editor's Manual
Free learning resource on English grammar, punctuation, usage, and style.
Both “who” and “that” can refer to people. “Who” is preferred in formal usage.
Use masculine and feminine pronouns like “he” and “she” instead of “it” for animals with names or whose sex is known. To refer to a species or an animal whose sex is not known, use “it” instead.
Use “who” for animals with names, animals that are personified or anthropomorphized, or to indicate emotional closeness. Use “which” or “that” to discuss a species or an unknown animal without a name.
Use “who” as a relative pronoun to link a description to the person it describes. “Who” is used not just for people but also animals with names. “Who” can replace “whom” in informal usage.
Don’t use a comma before “who” when it presents information necessary to meaning (a restrictive clause). Do use a comma when “who” introduces an optional description (a nonrestrictive clause).
Use “each of” with singular verbs to refer to every one of a group separately. “Each of” may be followed by a plural, gender-neutral pronoun. In spoken English, “each of” is sometimes used with plural verbs to refer to an entire group.
When “each” is part of the subject of a sentence, it is used with singular verbs, except when it follows a plural noun. “Each” may be used with a plural pronoun in an indefinite reference.
Don’t use a comma with “that,” either as a relative pronoun or a conjunction. “Which” usually introduces an optional description, which you should enclose in commas. No commas are used if “which” introduces essential information.
Use “a” or “an” before an abbreviation depending on how it is pronounced, not written. If it starts with a consonant sound, use a; if it starts with a vowel sound, use an (“a NATO member” but “an NFT”).
Use “which” to introduce a description. As a relative pronoun, “which” connects a relative clause to the noun it describes. Differences exist between American and British usage.