一 that 引导主语从句
1.that 引导主语从句时，that 在从句中不充当成分，但不可以省略。
e.g. That she gets on well with her classmates is known to us all.
That he was chosen made us very happy.
2.为了句子结构匀称，有时it 被用作主语，that引导的主语从句则被置于主句的 后面。
e.g. It is known to us all that she gets on well with her classmates.
It is hoped that one day they will have enough animals to set them free.
类似结构常见的有如下几种：
⑴ It is clear/ certain/ likely/ right/ wrong/ surprising /true that …
⑵ It is a pity/ shame/ good idea/ no wonder that ...
⑶ It is said / told/ known / reported/ suggested/ believed/ hoped/thought that ...
⑷It seems/ happens that ...
二. that 引导宾语从句
1. that引导宾语从句时，that 在从句中不充当成分，一般可以省略，但在以下几种情况下不能省略：
⑴ 当that 从句和主句谓语动词之间有插入语或者从句主语之间有插入语时, that 不可省略。
e.g. He announced ,believe it or not, that he would never forgive me.
He judged that, because she was a woman, she did not understand wine.
⑵ 当 that 从句与另一名词性从句并列作宾语时，that不可省略.
e.g. Everyone could see what was happening and that she was really worried.
⑶ 当 that 作介词宾语时，that 不能省略.
e.g. The reason lies in that he works harder than anyone else does.
2.为了句子结构匀称，有时it被用作形式宾语，that 引导的宾语从句被放在主句的后面.
e.g. They made it a rule that they had a party at the end of every month.
I think it necessary that you master a foreign language.
三. that 引导表语从句
that 引导表语从句时，that在从句中不充当成分，但不可以省略.
e.g. The reason why he was absent was that he was badly ill.
The problem with the project is that we are short of money.
四. that 引导同位语从句
1. that 引导同位语从句时，that在从句中不充当成分，但不可以省略.
e.g. I just got word that he is not coming this evening.
There is no doubt that you are in danger.
2. 为了句子结构匀称，有时同位语从句和它前面的名词会被谓语动词隔开：
e.g. The whole truth came out at last that he was a wolf in sheep's clothing.
Word came that another new research center would soon be set up.
3. that 引导同位语从句和 that 引导定语从句的区别
⑴ that 引导同位语从句时，只起连接作用，在从句中不担当任何成分；that 引导的同位语从句和它前面的名词之间是同位语关系，是具体说明前面名词的内容或含义的（同位语从句前面的名词常见的有：news, fact, thought, suggestion, truth, plan, belief, doubt, information, possibility, word, idea, order, hope, result等。）
⑵ that引导定语从句时，that 在从句中作主语或宾语；that引导的定语从句和它前面的先行词之间是修饰与被修饰，限制与被限制的关系：
e.g. We all heard the news that our team won the game.(同位语从句)
We all heard the news that was broadcast on the radio. (定语从句)
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