1.that引导的从句，无意义，不担当句子成分，不省略
(1)It +v+ that从句
(2)It +系动词+表语+that 从句
(3)It is strange that he didn't come here yesterday：奇怪的是，昨天他没来这里。
当形容词为important necessary strange /that should +动词原形，It is strange that he should （表示吃惊）pass the exam：奇怪的是，他可以通过考试
2.It is +n +that 从句
如：What a pity it is that you didn't come to this party：真遗憾你没来这个聚会。当名词为a pity/a shame /a surprise +should +v原形（表示竟然）
3.It is +过去分词+that 从句
如：It is reported that a car accident happened there：据报道，发生一场车祸
4.It +动词原形seem/appear /turn out/+that
如：It seems that it will rain：看来要下雨
(1)It strikes sb. that 从句 ：表示“某人突然想…”
如：It occurs to sb. that从句：表示“某人发生了…”
(2)为强调从句意义，that从句常放在句首
如：
That he will come here is of great help：他来这儿是很有帮助的
That you failed the exam will…：你考试失败了会…
(3)连接代词 who /whom/whose/what/which/担当从句成分，有意义，不省略
what +sb. + clause
如：What we will do next is not decided.我们接下来要做的还没有决定。
what+修饰词 意为什么样的……
如：What color you like is none of my business.你喜欢什么颜色不属于我的事。
what ……人，……事，……物
如：What he said at the meeting surprised everyone present.他在会议上说的令人惊讶。
