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)
追问清楚了，谢谢。
追答也欢迎直接找我提问，希望能尽力帮你解决英语学习上的问题。谢谢你的好评！