It is two weeks since I have seen him.( have seen-- saw)

问题描述:

It is two weeks since I have seen him.( have seen-- saw)
谁能够帮我讲出一个所以然?
麻烦题目看清啊——就是把HAVE SEEN改成了SAW 这是为什么?什么理由?

since引导的状语从句必用一般过去时态.例如,Two years has been since I came to Shanghai.