Grammatical; relating to grammar, especially whether something is grammatically correct or not.
その表現は文法的に正しいが、少し古風に聞こえる。
That expression is grammatically correct, but it sounds a bit old-fashioned.
文法的誤りを赤で修正してください。
Please mark grammatical errors in red.
文法的には問題ないが、会話では別の言い方が自然だ。
There is no grammatical problem, but a different phrasing is more natural in conversation.