Grammarian (noun)

1. A person who is an expert in the study of grammar.
2. A person who is skilled in the study of the rules and principles of a language or language in general.

Origin:

Late middle english: via latin from greek grammatikos "versed in grammar", from grammatikē (tekhnē) "(art) of letters", from gramma "letter".

Examples:

1. He was a renowned grammarian and a professor of linguistics.
2. The grammarian was able to explain the most complex grammar rules in a simple way.
3. The grammarian was correcting the manuscript for grammatical errors.
4. He was a grammarian, his expertise was in the Latin language.
5. The grammarian was explaining the difference between a gerund and a participle.