A grammatical person is a person who teaches or who is expert in grammar.
teacher
Penny teaches Baby
Teacher, professor, and instructor are some terms for someone who teaches.
An educator is someone who teaches or learns from someone else.
heretic
... is someone who teaches you about religion.
paster!:) or Father:)
a `teacher' preferbly
a botanist or a horticulturist
Answer They sometimes can be called an expert, a witness, or an expert witness.
A person who is an expert in grammar is called a linguist or a grammatician.