"掌握语法知识" 英文可以说 "master grammar knowledge" 或 "have a command of grammar."
来源："掌握" 的意思是 "to hold or control something in your hand"，"to have control or mastery over something." 而 "语法知识" 的意思是 "knowledge of the rules of a language." 因此 "掌握语法知识" 的意思就是 "to have control or mastery over the rules of a language."
英文例句：
She has a command of grammar that is impressive.
(她掌握语法知识的能力令人印象深刻。)
He mastered grammar knowledge and was able to speak fluently.
(他掌握了语法知识，能说得流利。)
中文翻译：
她掌握语法知识的能力令人印象深刻。
他掌握了语法知识，能说得流利。