grammarian

noun

1. A person who studies grammar, either scientifically or (traditionally) unscientifically.

   “Huddleston and Pullum are well-known grammarians of the 21st century.”

2. (broadly)A person who pedantically critiques the grammar of others' speech or writing, especially when unscientifically and misguidedly.

   “They spent hours disabusing the misinformed rants of various armchair grammarians.”