省略：
that引导宾语从句，起连接主、从句的作用，本身无意义，在口语或非正式文体中常省略。
不省略：
1）从句的主语是that时，that不能省。
We know that that is an important meeting．
2）动词后接多于两个由that引导的宾语从句时，第一个that可省，但后面的that不可省。
He said (that) you were too young to understand the matter and that he was asked not to tell you.
3) that引导的宾语从句中紧跟复合句从句时，that不能省。
I think that if you have lost the library book, you must pay for it．
4) 当主句的谓语与宾语从句之间有插入语时，that不能省。
He says, in the newspaper, that English is getting more and more important．
5) 当that从句是双宾语中的直接宾语时，that不可省。
I can’t tell him that his mother died.
that引导宾语从句的用法
1）常用在一些动词后面，这些动词有：say, think, insist, wish, hope, demand, imagine, wonder, know, suppose, see, believe, agree, admit, deny, expect, explain, order, command, feel, dream, suggest, hear, mean, notice, prefer, request, require, propose, declare, report等。
My father says (that)he is listening to the weather report．
2）主句是“主系表（形容词）”结构，形容词后常接that引导的宾语从句。这类形容词多表示感情：happy, glad, pleased, sad, sure, afraid等。
I’m afraid（that）he can’t come．
特殊用法：
否定转移：当主句谓语动词是 think, believe, suppose, expect 等词，宾语从句的意思是否定时，常把否定形式转移至主句表示。
I don’t think it is right for him to treat you like that.
