IPA: /ɡrəˈmɛəriən/
KK: /græˈmɛr.i.ən/
A person who studies or is an expert in the rules and structure of a language.
The grammarian explained the complex rules of punctuation to the students.
Grammarian → It is formed from "grammar" (from Latin *grammatica*, meaning the art of letters or writing) and "-ian" (meaning relating to or characterized by). The word "grammarian" refers to someone who is skilled in or specializes in grammar.
Think of a person who is skilled in the 'art of letters' ('grammar') — that's what a grammarian is.