1. 定义：句子的主语部分变成一个句子，充当主句的主语；
2. 形式主语it：主语从句中为了平衡句子结构，避免“头重脚轻”，常用it来充当形式主语，将真正的主语后移；
3.引导词：
(1)定义：连接主句和从句的词；
(2) 连词：只起连接作用，不作成分，包括that(无意思), if(是否)和whether(是否)；
(3) 关系代词：在从句中作主语，宾语或定语等成分；有what (事情或样子), which, who, whose等；
(4)关系副词：在从句中作状语，有where, when, why, how等；
(5)关系词：关系代词和关系副词统称为关系词，关系词在从句中作成分。
4. that引导的主语从句：既可放在句首，也可放在句末。但是为了保持句子的平衡，常用it作形式主语，把真正的主语从句放在句后；
e.g. That he is a doctor is true.
=It is true that he is a doctor.
It is surprising that she is so angry.
That he survived the accident is a miracle.
=It is a miracle that he survived the accident.
5. 主语从句中固定的四种搭配：
(1) It +be +名词+ that从句；
e.g. It is a fact that…
It is common sense(常识) that…
It’s a pity/a shame/a question/ an honor/a wonder…that…
(2) It + be +形容词+ that从句；
e.g. It is certain/clear/evident/likely/probable/better/natural that…
(3) It + be +过去分词+ that从句；
e.g. It is said/demanded/believed/reported/estimated that…
(4)It +不及物动词+ that从句。
e.g. It seems/happened/follows/matters/occurred to me/struck to me that…
6. whether与if引导主语从句的区别: whether引导的主语从句既可放在句首也可放在句后，if引导的主语从句只能放在句后。
e.g.Whether she comes or not makes no difference.
It is doubtful that whether the president knows the details of the plan.
7. wh-疑问词引导的表语从句：wh-疑问词引导的主语从句别表示具体的地点，时间，原因，方式，事情等。其中what (事情或样子), which, who, whose等为关系代词，作主语，宾语或定语等成分；where(地方), when(时间), why(原因，理由), how(方式)等为关系副词，作状语;
e.g. What he did yesterday is worth praising.
Why Tom decided to do the thing is reasonable.
8.“特殊疑问代词+ever”可以引导主语从句，表示“无论……”;但是“no matter +特殊疑问代词”不能应到主语从句，通常用于让步状语从句中;
e.g. Whoever did the thing deserves praising.
9.判断主语从句所用引导词的技巧；根据句子成分及句意理解；注意主语从句要用陈述语序。
10. 语法填空：
(1) It doesn't matter ________ you pay by cash or credit card in the store.
(2)_________ makes the book so extraordinary is the creative imagination of the writer.
(3) ________the delayed flight will take off depends much on the weather.
(4) It remains to be seen_________ the newly formed committee's policy can be put into practice.
(5) It suddenly occurred to him________ he had left his keys in the office.
(6) _______fashion differs from country to country may reflect a cultural difference.
(7) _______ she couldn't understand was that fewer and fewer students showed interest in science.
(8) __________has helped to save the drowning girl is worth praising.
(9) __________book you borrow, you must return it in a week.
(10) ________will go to the concert is not known.
(11) _______ he can do it is a problem.
(12) _______he was turned down the invitation was not clear.
(13) ________kinds of books they will buy needs considering.
11.改错：
(1)What is required in the regulations that you should not tell people the password of your email
account.
(2) If we'll go camping tomorrow depends on the weather.
(3) Where shall we spend the holiday hasn't been decided.
(4) That is certain that we can win.
(5)No matter who leaves the room last ought to turn off the lights.
(6) It is not certain if the policemen will come or not.
答案见于公众号初高中英语学习资料
