Yesterday, as I began a compressed review of grammar with the students in my editing class, I explained that not everything they have been taught as a rule qualifies as such. The Rules are not of a piece, and I went over these categories, which I first described in a post in 2012.
Explicit rules: Many of them are taught in composition classes, but they are often complex, with many exceptions and variations. Try to explain subject-verb agreement to a non-native speaker, making clear how "Ham and eggs are my favorite breakfast combination" and "Ham and eggs does not constitute a healthy breakfast" are both grammatically acceptable.
Conventions: Writers in the eighteenth century regularly inserted a comma between the subject and verb in a sentence. We don't any longer. English orthography is a swamp of maddening conventions. And they, too, like fashions, are subject to change; we no longer write to-day and to-morrow. But they are just conventions, not rules embedded in the language.
Superstitions: The things Arnold Zwicky calls "zombie rules"—no stranded prepositions, no split infinitives, none always a singular—have been exploded and rejected, not only by the descriptivists, but by a multitude of informed prescriptivists. Arnold Zwicky and Bryan Garner stand shoulder to shoulder against them. And yet, given a ghastly immortality by decades of defective schoolroom instruction, they persist against all reason.
Shibboleths: There are many usages that are not wrong but which people seeking advantage over their fellows preen themselves on avoiding. Ain't is a classic example. The hopefully peevers are gradually ascending the golden staircase, and in a few more years hopefully as a sentence adverb will be as unremarkable as contact as a verb (that extinct shibboleth from the 1950s). Since gaining minor advantage over one's fellows is an unending game, fresh shibboleths will continue to surface.
House style: These are the narrow conventions that individual publications or publishing houses insist on. They are very specific for legal, medical, scientific, and technical publications. And, of course, there are those sad souls on copy desks who think that the Associated Press Stylebook belongs on the shelf with the statutes of Hammurabi and Justinian. AP style is not inherently superior or inferior to Chicago style. A house style merely indicates that when choices are available, we do it this way for the sake of consistent practice, to avoid distracting the reader. A house style is a specific set of conventions, no more.
Individual aesthetic preferences: Everyone has them, and tinpot authorities—managing editors, self-appointed Guardians of the Language, that ilk—love to impose them on the weak and unwary. You happen to dislike impact as a verb and impactful as an adjective? Fine. Don't use them. But be hesitant about imposing your tastes as fiats on others.
Finally, a word to those of you wearing those "I judge you when you use poor grammar" T-shirts. Those of us who are former English majors will never experience wealth or power, and comeliness is probably right out as well, so we take what advantage of our bookishness that we can. But expressing judgments about other people's English usage is like commenting on their clothing: best not to express unsought opinions.