It is/has been + since sb +

It is/has been + since sb + It is/has been+since sb+did 怎么用? it has been since.用法 it is 3 years since Tom has been abroad It is a week since he has been to BeiJing. it is some time since+...与it has been some time since+...什么区 it is 时间段since从句和it has been时间since从句的区别 为啥it is some time since与it has been some time since没区别 句型 it is ...since it has been .since 还有意思一样的么?还有下面这道题 it is + 时间段 + since…和It has been+ 时间段 + since的问题 It is\has been+时间段+since+一般过去式=时间段+（）+since+一般过去式 it is/has been+时间段+since since后面接完成时还是一般过去?