一道高级英语的问题.It is a long time now since 的语法形式和用法?

It is +一段时间+since…的句型与It has been +一段时间+since…意思相同.前者是规范说法,后者是口语说法.应注意的是,在这种结构中,since引导的从句绝对不能用否定式的谓语动词来表示否定意义,下面两句都错了：It is a long time since he didn’t study English.It’s two months since you didn’t come to see me.正确的说法为：It’s a long time since he studied English 他好久没学英语了.You haven’t come to see me for two months.你有两个月没来看我了.

这是个常考的句式。如 It's a long time since we met last time.