As I go over some of the basics iin grammar with my undergraduate charges, I have tried to explain to them that the rules (which they no doubt think of as The Rules) are not all of a piece. Here is a slight revision of a taxonomy I posted early in 2012 about the categories of things that people treat as rules.
Actual but unnoticed rules: 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 never think about. The order of adjectives (article, observation, shape and size, color, origin, material, qualifier) is one; the idiomatic pairing of certain prepositions and certain verbs is another. You learned most of this before you ever had an English class, and you never have to think about it. It’s all in your head.
Actual but explicit rules: There are many of them, the things that we have to be taught for formal writing, 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, are subject to change; we no longer write to-day and to-morrow. These are fashions, not the basics of grammar and usage.
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 descriptivist tribe, but by a multitude of informed prescriptivists. And yet, given a ghastly immortality by generations of defective schoolroom teaching, they persist against all reason, eating into people’s brains.
Shibboleths: There are many usages that are not wrong but which people seeking status advantage over their fellows preen themselves on avoiding. Ain’t, I suppose, is the classic example, hopefully as a sentence adverb a comparatively recent addition—though the peevers maintain 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. They make sense for internal consistency and uniform tone for a publication. But no stylebook is inherently better or more correct than another. 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.
Sound practices: Parallel construction is not something that grammar requires, but it establishes patterns that enable the reader to pick up meaning more readily. A wordy sentence can be as grammatical as a concise one, but, again, concision is usually a benefit to the reader. Jargon can be as grammatical as plain speech, but it’s a good idea to keep it to the minimum necessary.
Individual aesthetic preferences: Everyone has them, and everyone is entitled to them. But they are merely expressions of personal taste and prefernce, and it is only tinpot authorities—managing editors, self-appointed Guardians of the Language, that ilk—who attempt to impose them on the weak and unwary.