What is the difference between a style guide rule and a grammar rule?

A grammar rule describes how the language actually works. A style guide rule is a house preference chosen for consistency. Confusing them causes a great deal of unnecessary argument, because people defend preferences as though they were laws.

Genuine grammar is largely invisible to native speakers because they never break it. "The cat sat on the mat" is grammatical; "cat the mat on sat the" is not. Nobody needs a guide for this — it is internalised. Real grammatical errors in adult writing are mostly slips: agreement failures in long sentences, dangling modifiers, mismatched constructions.

Style choices are conventions with no correct answer:

Oxford comma or not. Per cent or percent. Numbers spelled out below ten or below one hundred. Single or double quotation marks. Whether to capitalise job titles. Serial hyphenation. Date formats.

Each of these has a defensible case both ways. What matters is picking one and applying it consistently, which is exactly what a style guide is for — it is a decision record, not a rulebook.

The third category is the trouble: invented rules. Several widely taught prohibitions were never features of English:

Never split an infinitive — imported from Latin, where infinitives are single words and cannot be split.

Never end a sentence with a preposition — a seventeenth-century preference, not a rule.

Never start a sentence with and or but — done by good writers for a thousand years.

Never use the passive — the passive is correct and often the right choice when the actor is unknown or irrelevant.

How to tell which you are dealing with: ask whether breaking it makes the sentence unintelligible (grammar), inconsistent (style), or merely displeasing to someone (a preference).

In professional writing, follow whichever guide your organisation uses and keep your own decisions in a style sheet.