The Editor’s Manual
Free learning resource on English grammar, punctuation, usage, and style.
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).
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 “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.
Use “that” as a relative pronoun in restrictive or defining clauses, which present information essential to meaning. Don’t use a comma before “that.”
A compound subject is made up of two or more subjects that share the same predicate (“The dog and the cat sat on the rug”). Use pronouns and verbs correctly with compound subjects.
Anticipatory reference occurs when a pronoun appears before its antecedent, or the person or thing it refers to, in a sentence (“When she can, Rita runs marathons”).
Use a semicolon in place of a period to join two closely related sentences. Also use it to separate list items that themselves contain punctuation.