1.that引导主语从句that引导主语从句时不同于其他的连词，既无词义也不作成分，只起单纯的连接作用，且通常不可以省。为了强调that引导的主语从句而置之于句首时，that不可省。①That the driver could not control his car wasobvious.②That she was chosen made us very happy.③That he will come is certain.④That he would take the risk is true.⑤That he should have married her isn't surprising.
★通常，that引导主语从句时用it做形式主语，把从句置于句尾，此时that有时可省。例如：①It was obvious that the driver could not control his car.②It made us very happy that she was chosen.③It is certain that he will come.④It is true that he would take the risk.⑤It isn't surprising (that) he should have married her.
★如果以that从句为主语的句子是疑问句，就只能用先行词it结构。例如：①Is it certain that he will come?②Is it true that he would take the risk?
▲it做形式主语有以下几种不同的结构：(1)It + be + 形容词+ that-从句①It is likely that he will come.②It is strange that she has ever trusted him.③It is important that he (should) attend the meeting.④It is best that he (should) go.
此句型在表示惊讶、喜悦、遗憾等感情色彩时，that从句中的谓语动词也常用 (should) + v原形。It is strange that no one should haveobjectedto the plan.
(2)It + be +名词+ that-从句It is common knowledge that the whale is not a fish.It's a pity that he should havecatched the train.
(3) It + be + -ed分词 + that从句It is said that he is a famous writer.It is reported that there will be a storm this afternoon.
▲在表示建议、要求、命令等意义的被动结构中，that从句中的谓语动词常用(should) do。（参见虚拟语气部分）It is suggested that the meeting (should) be held this afternoon.It is demanded that he (should) leave at once.
(4) It + seem/happen/appear等不及物动词 + that从句。（look不接that从句，它接to be结构或形容词）It seems that he is wrong.It appears that they are in urgent need of help.He looks (to be) surprised/happy/sad.
2. whether引导主语从句，意为“是否”，只起连接作用，不充当任何成分。（置于句首时必须用whether引导，置于句尾时，间或可用if。）Whether he will win the game is not clear.Whether he will come isuncertain.Whether she comes or not doesn't concern me.It is not clear whether/if he will come.It was uncertain whether he would come.
3. 连词代词what, who, which, whose等引导的主语从句。这些连接代词作用相当于代词，在从句中担当主语、宾语、表语、定语等。What we need is more time.Who wasresponsiblefor the accident is not yet clear.Whose book it is not important.Which school you want to go matters much.
4.连接副词when, where, why, how引导的主语从句。这些连接副词相当于副词，在从句中充当状语。Why he did this is not known.When he will come is still unknown.It is not known why he did this.How he succeeded is unknown to us.Where we should hold the meeting needs to be discussed.
5.也可用whatever,whoever,whomever,whenever,wherever等词引导主语从句，表示“无论什么”、“无论谁”、“无论何时”、“无论在（到）哪里”等。Whoever told you to give up smoking was quite right.Whatever he gave you should be handed in.Whoever told you that waslying.Whoever comes is welcome.