分析一下它的含义?It is has been four years since he lived here．

这句话的意思是：他已经有四年不住在这里了.若since引导的状语从句的谓语动词是终止性的过去时,则从句表示的时间是“从动作开始的那一时刻起”.如：
We have been missing them since they left here自从他们离开这里,我们就一直很想念他们.若since引导的状语从句的谓语动词是持续性动词或表示状态的动词的过去时时,则从句表示的时间是“从那持续动作或状态结束时算起”.如：
John is now with his parents in New York,it is already three years since he was a teacher约翰现在和父母一起住在纽约,他不当教师已经三年了.
How long is it since you lived in Shanghai?你离开（没住在）上海多久了?