Lily was admitted by a famous unversity in 1990,since which year we have never heart from him. 1、直接改成since which 用which自己代1990行么？答：不行。因为which在此【做定语】修饰year。2、改since when 成么？答：不成。因为since是介词，后面不能接副词或连接副词。3、为何不能使用after which year？答：因为since引导的时间是【从过去延续到现在】，刚好与【现在完成时have never heart from him相吻合】。...
