First, let’s be clear about what “usage” means. It isn’t grammar, though the two are related and often treated together in such page-turners as The McGraw-Hill Handbook of English Grammar and Usage and The Chicago Guide to Grammar, Usage, and Punctuation. Grammar is a language’s set of rules for putting words together; usage is how people actually put words together. Remember it this way: usage is how language is used.
So Bryan A. Garner didn’t release a fifth edition of his Modern English Usage to explain the difference between coordinate and subordinate clauses—that hasn’t changed since 2016, when the previous edition was published. He’s here to update us on changes in how people speak and write in real life. Six years ago, for instance, using “they” as an alternative to “he” or “she,” Garner wrote, “sets many literate Americans’ teeth on edge.” As his new guide notes, the Chicago Manual of Style, New York Times, and Associated Press all now allow it.
To chart such evolutions, he’s devised a “language-change index,” consisting of five stages, from the emergence of a variant to its widespread acceptance. Which stage a word belongs in isn’t always clear. He tentatively puts “they” as a third-person singular in Stage 5, but given his description of Stage 4 usage—generally adopted but “opposed on cogent grounds by a few linguistic stalwarts”—you could argue it’s the latter. Surely it draws at least as much opposition as other Stage 4 specimens: “impact” as a verb and “hone in” for “home in.”