it is 时间 since 延续性v.,从句为何表否定?

一般中学阶段since引导的状语从句里不讲使用延续性动词,看来你接触到了,那么就简单明了介绍一下,希望你能理解清晰：
看下面两句话：
He has been working in Guangzhou since he left Shanghai.(这个很直接很直观好理解）
He has been working in Guangzhou since he was in Shanghai.（这个从句中be是延续性动词,它表达的意思是“自从他（过去还）在上海到现在“,其实时间就是从他还在上海的最后一刻算起,即a几乎等于”离开“那一刻.再比如：
I have worked at school since I was a college student(=since I graduated from college)