that在名词性从句中，只起连接作用，在从句中不作任何成分，不含任何意义，引导名词性从句（除动词后的宾语从句外）时不可省略，故称作“三不”。
1. that引导主语从句、表语从句、同位语从句时，不能省略。
The most pleasant thing of the rainy season is that one can be entirely free from dust.雨季最令人愉快的事是人们可以完全摆脱灰尘。
2. that引导动词宾语从句时一般可省略。
My uncle is the owner of a restaurant close to where I live.我的叔叔在我家附近有一家餐馆。
3.还有以下几种情况that不能省略。
当that引导的宾语从句和主句的谓语动词之间有插入语或者從句主语之后有插入语。
如：He told us that if we could all come to help out， the whole situation would be better.
当一个动词引出两个（或以上）的并列的宾语从句时，that不能省略。
如：Some believe （that） it’s easy to learn a new language and that practice makes perfect.
当that引出的从句作介词宾语时不能省略。
如：The reason lies in that she works harder than others.
