Formal:
* Grammarian: This is the most common and direct term.
* Linguist: While linguists study language in general, those specializing in syntax and morphology could be considered grammar experts.
* Language expert: This is a general term that can encompass grammar expertise.
Informal:
* Grammar whiz: This implies a high level of skill and knowledge in grammar.
* Grammar guru: This is a playful term for someone who knows a lot about grammar.
* Grammar nerd: This is a humorous term for someone who enjoys and is very knowledgeable about grammar.
The best term to use depends on the specific context and the tone you want to convey.