介绍:
It has been (is) +一段时间+since从句
自从.做什么事情以来已经有很长时间了
例句：
我们已经5年没见面了
It has been 5 years since we last met.
自从他离开家乡已经由15年了。
It has been 15 years since he left his hometown.
It is 3 years since I lived in Shanghai.
自从我离开上海已经3年了。
since 后面出现的是延续性动词时，我们应理解为自从该动作结束有多久了。
