__he entered the university,his english has been much improved.A after B since C before
问题描述：
__he entered the university,his english has been much improved.A after B since C before
A和C用于怎样的句子中?
答
after和before如果置于句首,有两种情况,一是充当介词角色,它们后面接的是动名词,即动词的ing形式；二是作为连词,引导的是从句,后面的主句应该用一般过去式.