adj. grammatical, of or pertaining to grammar, pertaining to the study of classes of words, pertaining to the study of the structure of a language; conforming to the rules of grammar

nm. grammarian, one who specializes in grammar

adj. grammarian, one who specializes in grammar

n. grammarian, one who specializes in grammar

(n.) = grammarian.
Ex: A reader or a listener, the descriptive grammarians would argue, would not assume, despite the singular verb and the objective case pronouns, that Mary was the subject, rather than the object, of the sentence.