常考的强调句结构是it 引导的句子。
    It is (was) 被强调部分 that (who) 句子其他部分。
    
    
    此结构强调的成分仅限于主语，宾语和状语。
    It is from the sun that we get light and heat.
    It was not until I had read your letter that I understood the true state of affairs.
    
    
    典型例题
    1） It was last night ___ I see the comet.
    A.  the time  B.  when   C.  that    D.  which
    答案C. 强调句的结构是： It be 强调部分 that (who) 主谓句。  强调句的连词只有两个，that和who。当强调的部分是人，且为句子的主语时，才用 "who"，其余用that。
    
    
    原句：  My father did the experiment in the lab yesterday evening.
    强调主语： It was my father who did the experiment in the lab yesterday evening.
    强调宾语： It was the experiment that my father did in the lab yesterday evening.
    强调时间： It was yesterday evening that my father did the experiment in the lab. (注意不用when)
    强调地点： It was in the lab that my father did the experiment yesterday evening.
    
    2）It is ten years ___ Miss Green returned to Canada.
    A. that  B. when   C. since   D. as
    答案C. 考点是连词用法。 本题易误选为A. that. 其实本句不是强调句。若是，去掉It   be… that还应是一个完整的句子。而本句去掉 'It is…that'，只剩下ten years Miss Green returned to Canada. 不成句。因此本句不是强调句。
    It is /was 时间 since…  其中is<---> has been   was <---> had been.