Neutral:
* Grammar Nazi: This is a common and somewhat humorous term, but can be seen as offensive by some.
* Grammar police: Similar to "Grammar Nazi," but slightly less aggressive.
* Grammar enthusiast: This term is neutral and emphasizes their interest in grammar.
* Pedant: This word suggests someone who is overly concerned with minor details and rules.
Negative:
* Nitpicker: This implies someone who focuses on trivial errors.
* Obsessive: This suggests an unhealthy preoccupation with grammar.
* Pretentious: This word implies that the person is trying to show off their knowledge.
Positive:
* Grammar expert: This term is respectful and highlights their knowledge.
* Grammar aficionado: This implies a passionate love for grammar.
The best word to use will depend on the specific situation and your relationship with the person.