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】.