It is (was) 被强调部分+ that (who) + 句子其他部分。
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.
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.
强调句 - 常用句型
It is/ was + 被强调部分（通常是主语、宾语或状语）+ that/ who（当强调主语且主语指人）+ 其他部分。
例子 It was yesterday that he met Li Ping.
例子 Was it yesterday that he met Li Ping?
被强调部分（通常是疑问代词或疑问副词）+ is/ was + it + that/ who + 其他部分？
例子 When and where was it that you were born?
4.not … until … 句型的强调句
A. 句型为：It is/ was not until + 被强调部分 + that + 其他部分
普通句：He didn't go to bed until/ till his wife came back.
强调句：It was not until his wife came back that he went to bed.
此句型只用until，不用till。但如果不是强调句型，till, until可通用；因为句型中It is/ was not ... 已经是否定句了，that后面的从句要用肯定句，切勿再用否定句了。
A. It is/ was ... that ... 结构不能强调谓语，如果需要强调谓语时，用助动词do/ does或did
Do sit down. 务必请坐。
Do be careful when you cross the street. 过马路时，务必（千万）要小心啊！