It+时间+从句

请看下面一道题:

The field research will take Joan and Paul about five months; it will be a long time _____ we meet them again. A. after B. before C. since D. when

答案选B.这是2007年安徽的一道高考题，其实质上是通过“it + be+从句”这一句型来考查了连词的用法。在该结构中连词的选择是有规可循的，主要是由主句、从句中的谓语动词的时态来确定的。下面通过下表比较（尤其是对主从中时态的比较）就能很容易的掌握。

从句中谓含义 例句 语动词的形式 It is +时间段+since一般过去（到现在为止）做It is three years since the 从句 时 某事已有多长时间war broke out. 了 It was+时间段过去完成（到过去某时为It was two years since he +since从句 时 止）做某事已有多had left here. 长时间了 It was +时间段一般过去过了多长时间It was a long time before I +before从句 时 才„„ got to sleep again. It was not +时间段一般过去没过多长时间It wasn’t long before he +before从句 时 就„„ told me about this affair It will be +时间段一般现在要过了多长时间It will be five years before +before从句 时 才„„ we meet again. It will be not +时一般过去没过多长时间It won’t three hours 间段+before从句 时 就„„ before he arrives at the bus stop. It is/was +时间点 做某事的时间是什It was eight o’clock in the +when引导的定语从么时间 morning when he went 句 shopping. It is/was+时间状语 正是在某个时间，It was at eight o’clock in +that引导的强调句 某事发生了 the morning that he went shopping. It’s time that+从一般过去是该做某事的时候It is time that I went 句 时（或了 (should go) to pick up his should+daughter. 动词原形） 结构 It is the 现在完成first/second/third 时 time +that从句 It/This/That was 过去完成the 时 first/second/third time +that从句

某人第一/二/三次It is my first time (that) 做„„ I have visited the Great Wall. 某人第一/二/三次It was the second time that 做„„ the film had been shown in this cinema