Formal:
* Linguist: This is a broad term for someone who studies language, but it can encompass grammar experts.
* Grammarian: This is the most direct and specific term for someone who studies and is an expert in grammar.
* Syntax Specialist: This term is used for someone who specializes in the structure and arrangement of words in sentences.
* Language Professor: This term is used for a university professor who teaches and researches language, including grammar.
Informal:
* Grammar Guru: This term is informal and often used humorously to describe someone who knows a lot about grammar.
* Grammar Whiz: This term is also informal and used to describe someone who is very good at grammar.
* Language Maven: This term is a bit more formal than the previous two and is used to describe someone who is an expert in language, including grammar.
The best term to use will depend on the specific situation and the level of formality you want to convey.