Neutral:
* Grammar stickler: This is a common and neutral term.
* Pedantic: This implies that the person is overly concerned with minor details and rules, often to the point of being annoying.
* Nitpicker: A more general term for someone who focuses on minor flaws.
* Grammar enthusiast: This emphasizes their interest in grammar, without implying negativity.
Negative:
* Grammar Nazi: This term is often used in a derogatory way and implies that the person is overly critical and inflexible.
* Language purist: This implies that the person is overly strict about adhering to traditional language rules, often to the point of excluding or rejecting new usages.
* Hypercritical: This describes someone who is excessively critical, particularly of minor details.
Positive:
* Grammar expert: This implies that the person has a deep understanding of grammar and is able to identify errors.
* Linguistic purist: This implies that the person is dedicated to preserving the purity and correctness of language, but in a more respectful way than "language purist."
The best word to use will depend on the context and your intended meaning.