A grammatical rule is a pattern statement about sentence form in a language variety.
The test: does the statement judge a form well-formed?
Word order: position matters. Dogs bite cats versus Cats bite dogs.
Inflection: She walks, not She walk.
Agreement: They are, not They is.
Main types and examples
Negation: a fixed negative pattern must hold, and the pattern varies by dialect. Standard English allows one negative: I don't know works, I not know does not. Some varieties use negative concord, so I can't get no satisfaction is well-formed there, but fails the standard variety's rule.
Questions: inversion marks one common question pattern. Are you ready? places the verb before the subject. A declarative form with rising intonation, like You are ready?, is well-formed spoken English too, so inversion is one option, not the only grammatical one.
Pronoun case: form changes with role in the sentence. She saw him uses one pair; He saw her uses another.
Tense: time is marked by verb form, not by a meaning note. I went yesterday is well-formed; I go yesterday is not, in standard English.
Determiners: a noun needs the right company before it. A book fits the pattern; a books does not.
The ambiguous case
Prescriptive rules look similar but fail the test: they state how people should write or speak, not whether a form is well-formed in a dialect.
- Avoid splitting infinitives. This prescribes a preference; to boldly go is well-formed in every measured variety, so it is not a grammatical rule in the descriptive sense.
If a statement bans a form that native speakers use routinely and no verdict changes across dialects, treat it as advice about register, not grammar.