1、and连接两个宾语从句，that宾语从句放在and的后面时，that不能省略。2、that引导的宾语从句作介词宾语时，that不能省略。3、that从句位于句首时，that不可省略。4、主句谓语动词与that从句之间有插入语，that不可省略。

例句

Everybody could see what happened and that Tom was frightened

大家都会看出所发生的事情并知道Tom非常害怕。

I know nothing about him except that he is from the south

对他我一无所知，只知道他是南方人。

That he ever said such a thing I simply don’t believe

我简直不相信他曾说过这样的话。

We decided, in view of his special circumstances, that we would admit him for a probationary period.

鉴于他的特殊情况，我们决定让他试用一段时间。