Whenever the conversation turned to his past, he would quickly change the subject.

change 是动词，意思是"改变、变换"。 the 是定冠词，表示特指某个事物。 subject 是名词，意思是"主题、话题"。 change the subject 是一个固定短语，意思是"转移话题"，指故意把谈话内容从当前主题转向其他方面。