I got into a discussion lately with someone about rules of English usage, and the need for greater clarity moves me to publish this summary, which I share with my students.
The rules of English usage are not of a piece, and there are different categories, with varying weights, among the things people think of as rules:
The dark grammar: Unless you are learning English as a second language as an adult, there is a whole network of subterranean rules of grammar that most native speakers internalized in early childhood and never think about. That subject-verb-object order is standard in English is one. The order of adjectives is another. You know that “the little red brick Gothic Revival church” is English and “the brick little Gothic Revival red church” is not.
Explicit rules: Many are required for the mastery of standard formal written English, a learned dialect, which is why they must be studied in school. (Standard formal written English is nobody’s native dialect.) 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: Punctuation and spelling are not rules but conventions of usage that are mutable. 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, are subject to change; we no longer write “to-day” and “to-morrow.”
Superstitions: The things Arnold Zwicky calls “zombie rules”—no end-of-sentence prepositions, no split infinitives, none always a singular—have been exploded and rejected, not only by the descriptivist tribe, but by a multitude of prescriptivists. And yet, given a ghastly immortality by generations of defective schoolroom teaching, 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, I suppose, is the classic example, hopefully a comparatively recent addition—though the peeververein maintains a vast and ever-increasing store.
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.
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 are entitled to indulge your own idiosyncratic preferences, but not to require them of others.