since的问题------ four years since John left school.请解释一下前面为什么用

It ___ 6 years since we left middle school. It _（be) two years since he left school. Three years ___(pass)since I left school. it has been three years since i left school（be away for）用括号内 it is four years since I joined the army.这里的is,为什么不用was呢? It's two years since he left his hometown.since 为什么不能换成after he has left since three years ago可以用吗 1、It____ten years since he left Shanghai. It __ten years since he left Shanghai. It ___ six years since I left school.空格填什么? Three years has passed___we left school.A.after B.since C.as 初二英语、练习选择It is ten years since Jim left school .ow he has __