有位朋友的笔记写到：
The point is you can only achieve Win/Win solutions with Win/Win processes -- the end and the means are the same.
这段话中的that最好不要省略，因为读者可能会读成 The point is you / can only而有所误解。
经典写作指南The Little, Brown Handbook中有类似的例子：
“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...."
关于that是否可以省略的问题并没有清楚的规定，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.
以英语为母语的人可以依靠直觉来判断是否可以省略that，对我们来说该怎么办？我们所谓的语感往往并不靠谱。Grammar Girl给了我们建议：
If you’re not a native speaker, I recommend keeping the "that" unless you’re dealing with a verb, noun, or adjective that you know will sound good without it. It’s safer to leave it in than to leave it out. As you write and read more, you’ll identify more of the words that allow you to omit "that".
也就是说，安全起见就不省略。只在我们确定如果省略that这句话会很自然、没有歧义的情况下才省略。那怎么知道哪些情况会“很自然”呢？只有多读、多观察。
我找了一些资料分享给大家，可能对你会有帮助。
Fowler's Modern English Usage中说：Omission of the conjunction that is acceptable, not only in informal contesxts. 也就是说是否省略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。
*另外下面说的that，指的是当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.
1. that前有什么样的动词一般可以省略？
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 thinkI’ll make a shopping list today. ✅
You saidyou 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.
上面的两个例句完全满足这三点要求，于是可以省略。
什么是bridge verbs呢？这个比较复杂我也说不清楚，我们姑且把他们认为是“单纯”的词，例如say就是“说”，hear就是“听到”, 这时候要强调的主要信息在后面的从句中。
而non-bridge verbs会复杂一些，例如whisper, quip都是non-bridge verbs, whisper不只是“说”，还强调了“小声地说”。如果谓语动词是non-bridge verbs的话，一般that就不省略。例如常见的有acknowledge, confirm, assert, contend, maintain，insist，order后面的that就不省略。
The department confirmedthere were some victims.
Mexican officials acknowledgethey are hampered by a lack of information.
牛津出版的Practice of English Usage中说: Thatcannot 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 repliedthat he was feeling better. ✅
James replied he was. ❌
She shouted that she was busy. ✅
She shoutedshe was busy. ❌
2. That前面是名词，一般都不省略
在名词后的that，除了possibility, feeling外，一般都不省略。例如:
I did not believe his claimthat he was ill. ✅
I did not believe his claimhe was ill. �� （不自然）
He disagreed with Copernicus' viewthat the earth went round the sun. ✅
He disagreed with Copernicus' viewthe earth went round the sun. ❌
3. That前面的形容词一般可以省略
对于比较普遍有的情感，例如glad, sad，一般可以省略that：
I’m gladyou came. ✅
We’re sadyou’re leaving. ✅
We were surprisedshe came. ✅
对于比较具体的情感，例如furious, ecstatic，最好不要省略。
很多时候，是否省略只是一个个人偏好。如张满胜老师在《英语语法新思维》中说: “语法规则就是对这些规律性的语言现象进行归纳和总结，但并不能涵盖所有的语言现象。从这个意义上讲，所谓的“语法”规则只不过是一种权益之计。” 我们能做的就是多读多观察，有一个开放的语法观，不把所谓的语法规则当作“不可逾越的清规戒律或天规”。
作家Neil Gaiman在回答写作中语法的重要性时说了这样一段话，我很喜欢，也分享给你：
Don’t obsess over grammar. If you have to obsess, obsess over clarity. Write as clearly as you can. When it works, there’s a magic in writing: you can get an idea out of your head and into someone else’s. That’s your goal."
