我们先来看看经典写作指南The Little, Brown Handbook中的一个例子：
“写作圣经”The Elements of Styles中也提到了这点：
The ear, for example, must decide when to omit that from a sentence, when to retain it. “She knew she could do it” is preferable to “She knew that she could do it” — simpler and just as clear. But in many cases the that is needed. “He felt that his big nose, which was sunburned, made him look ridiculous.” Omit the that and you have “He felt his big nose….”
很多同学不知道什么时候该省略什么时候不该省略。而关于that是否可以省略的问题官方也并没有清楚的规定，基本上靠跟着感觉走。如E.B. White和William Strunk Jr. 所说，是否省略得由the ear来决定。Bill Bryson也会同意他们的观点，他在Troublesome Words中说：
Whether you say “I think you are wrong” or “I think that you are wrong” is partly a matter of idiom but mostly a matter of preference. Some words usually require that (assert, contend, maintain) and some usually do not (say, think), but there are no hard rules. On the whole, it is better to dispense with that when it isn’t necessary.
Fowler’s Modern English Usage中说：Omission of the conjunction that is acceptable, not only in informal contesxts. 我们可以得出是否省略that并不是书面语法和口语语法的区别，正式文体中也是可以省略的结论。
还有下面说的that，指的是同学们在用that做“conjunction”（主从连词）的时候，也就是用that引导宾语从句或同位语从句的时候。当that做“关系代词”（也就是引导定语从句的时候），不充当主语的话一般是可以省略的，例如There are the people (that) we met in China.
遇到think, know, claim, hear, believe（语言学中称它们为bridge verbs) 时，that一般都会被省略。例如：
I think I’ll make a shopping list today.
You said you didn’t.
在朗文的语法书Longman Student Grammar of Spoken and Written English中有更具体的说明：
Three grammatical factors are associated with the omission of that:
1. say/think as the main clause verb
2. co-referential subjects in the main clause and that-clause
3. a personal pronoun as subject of the that-clause.
The department confirmed there were some victims.
Mexican officials acknowledge they are hampered by a lack of information.
牛津出版的Practice of English Usage中说: That cannot be dropped after certain verbs, especially instransitive verbs — e.g. reply, email, shout. 我认为这里说的”certain verbs”和上面的”non-bridge verbs”应该是同一类。
James replied that he was feeling better.
James replied he was.
She shouted that she was busy.
She shouted she was busy.
I did not believe his claim that he was ill.
I did not believe his claim he was ill. （不自然）
He disagreed with Copernicus’ view that the earth went round the sun.
He disagreed with Copernicus’ view the earth went round the sun.