Section Ⅲ Grammar —表语从句和主语从句语法图解探究发现①My feeling is that they're going to be big one day.②But the big surprise of the evening was when I was elected prom queen!③China is no longer what it used to be.④The reason why I'm calling you is that I want to invite you to my birthday party.⑤That she has became an artist may have been due to her father's influence.⑥What we should do next remains unknown.⑦Whether it will do us harm remains to be seen.⑧It's a good thing that the exams are finished.[我的发现](1)以上句中，①～④句中黑体部分均引导表语从句，而⑤～⑧句中黑体部分则引导主语从句。
(2)由④句可知，主语为名词reason时，表语从句中的连接词要用that，而不用why。
(3)由⑤⑥句知，引导主语从句时，what要在从句中充当句子成分，而that不作任何成分，只在语法上起连接作用。
(4)⑧句中，it用作形式主语，代替that引导的主语从句。
一、表语从句1．定义在复合句中作表语的从句，叫作表语从句。
它位于主句的系动词之后，对主语进行解释、说明，使主语的内容具体化。
2．引导词[即时演练1] 用适当的连接词填空①My opinion is that we should communicate with parents frequently.②The problem is whether we can find them there.③This is what I should have done first of all.④This is where I don't agree with you.⑤The question is how we can get in touch with him.⑥The problem is who will come to replace her.⑦This is because it is raining outside.3．注意事项(1)that与what的区别that引导表语从句时，是连词，在从句中不作成分；what是连接代词，引导表语从句时，在从句中作主语、宾语、表语或定语。
That was what she did this morning on her way to school.那就是她今天早晨在去学校的路上做过的事。
(2)because与why的区别because引导表语从句时强调原因，why引导表语从句时强调结果。
She often comes late.That's because she has a baby to take care of.她经常迟到。
那是因为她有一个孩子要照顾。
She has a baby to take care of.That's why she often comes late.她有一个孩子要照顾。
那就是她经常迟到的原因。
[名师点津] 名词reason作主语时，后面的表语从句表示原因时要用that引导，而不用because。
The reason why he was late is that it was raining hard.他来晚了是因为天正下大雨。
[即时演练2] 用that/what/because/why填空①(2016·北京高考改编)The most pleasant thing of the rainy season is that one can be entirely free from dust.②This was what he told me.③My belief is that I will succeed by working hard.④I had neither a raincoat nor an umbrella.That is why I got wet through.⑤You always make the same mistakes in the exams.I think that is because you are too nervous.(3)虚拟语气用于表语从句①在表示建议、劝告、命令等含义的名词后的表语从句中，谓语动词用“should＋动词原形”，should可以省略。
My suggestion is that we (should) have a discussion about this matter instead of just laying it aside.我的建议是我们应当讨论一下这件事，而不应该只是把它放在一边。
②as if/as though引导的表语从句常用虚拟语气。
用一般过去时表示现在，过去进行时表示现在进行，过去完成时表示过去，could/would/might＋动词原形表示将来。
It looks as if he were ten years younger today.今天他看起来好像年轻了10岁。
[即时演练3] 用所给词的适当形式填空①The doctor's advice is that we (should) take (take)more exercise.②The proposal is that the school (should) enlarge (enlarge) the playground.二、主语从句(一)主语从句的引导词连接词：that, if, whether；连接代词：what, who, whom, which, whose, whatever, whoever, whichever, whomever；连接副词：when, where, how, why等。
It is very important whether we can finish that task on time.我们能否按时完成任务很重要。
It is certain that the film Wolf WarriorⅡ was a success.可以肯定的是电影《战狼Ⅱ》是一部成功的影片。
Whichever of you comes in will receive a prize.你们当中不论哪个进来都将得到奖品。
[名师点津] whether/if在从句中不充当句子成分，起连接作用，意为“是否”。
whether引导的主语从句既可放在句首，也可放在句尾；if引导的主语从句只能放在句尾，前面用it作形式主语。
如果whether引导的主语从句用it作形式主语，whether可换成if。
Whether life exists on other planets is not clear.其他星球上是否存在生命还不清楚。
It is doubtful whether/if she would play the role.她是否要演这个角色还很难说。
[即时演练4] 完成句子①Whether he'll come here isn't clear.他是否会来这里还不清楚。
②That the earth turns around the sun is known to all.我们都知道地球围着太阳转。
③What will happen to the world is difficult to predict.世界将发生什么事情是难以预测的。
④Who will win the match is still unknown.谁能赢得这场比赛还不得而知。
⑤Why he gave up the chance of studying abroad is unknown.他为什么放弃了出国学习的机会还不清楚。
(二)注意事项1．常用it作形式主语的句型(1)It＋be＋形容词(obvious, true, natural, surprising, good, wonderful, funny, certain等)＋that从句It is wonderful that all of you have passed the difficult exam.你们都通过了这么难的考试，真是太好了。
[名师点津] 在“It＋be＋形容词＋that从句”句型中，如果形容词为necessary, important, strange, natural等时，从句谓语动词常用“(should＋)动词原形”。
It is important that we (should) build more hydroelectric power stations.我们建立更多的水力发电站是重要的。
(2)It＋be＋名词词组(no wonder, an honour, a good thing, a pity, no surprise, etc.)＋that从句It is no wonder that she was so upset.难怪她如此心烦意乱。
(3)It＋be＋过去分词(said, reported, thought, expected, decided, announced, arranged, etc.)＋that从句It's reported that another earth satellite has been put into orbit.据报道另一地球卫星已被送入轨道。
[名师点津] 在“It＋be＋过去分词＋that从句”句型中，如果过去分词为suggested, advised, ordered, requested, required等时，that从句谓语动词使用“(should＋)动词原形”。
It is suggested that you (should) attend the opening ceremony.有人建议你出席开幕典礼。
(4)It seems/happens等不及物动词或短语＋that从句It seems that they have known our secret.好像他们已经知道了我们的秘密。