历年高考单选题中常常出现It +be+时间+从句的这类句型，先看几道高考题：
例1：It is almost five years ________we saw each other last time.(05北京)
A. before B. since C. after D. when
正确答案：B 我们已经有5年没见面了。
例2：-Did Jack come back early last night？
-Yes. It was not yet eight o‘clock ________he arrived home.(2005福建24)
A. before B. when C. that D. until
正确答案：B 他回家时还不到8点。
这类考题出现频率较高，考生又极易混淆，在复习中做出归类非常必要：
1.It is/has been + 时间段 + since + 一般过去时
例1：It is/has been 10 years since he came here.这类句型可转化为：
=Ten years has passed since he came here.
这一句型可扩展为：It was+时间段+since(从句用过去完成时)
例2：It was years since he had arrived in China.(过去是+since+过去完成时)
需要注意的是since从句的谓语动词一般是非延续性动词，但有时也可用延续性动词或表示状态的词，此时需要注意句子的翻译：
例3：It is three years since she came to our class.她来我们班已经有三年了。