宾语从句或表语从句中that有时可以省略，但在以下情况时，that不宜省略。
一、当动词后带有两个并列的宾语从句时，引入第二个从句的连词that不可省略。例如：
The teacher told thestudents (that) they should hand in their homework the next dayand that the homework must be signed names by their parents.老师告诉学生他们要在第二天交上作业，并且作业必须由家长签名。
二、当that前面有插入语时，that不宜省略。例如：
Little Alice didn't know, I'm sure, that her sister was going to America. 我确信，小艾丽斯不知道她的姐姐很快就要去美国了。
三、省略答语中，that宾语从句单独出现时，that不可省略。例如：
—What did hesay? 他说了什么？
—That he would visit the Great Wall. (他说)他将会参观长城。
四、当主句的状语部分位于that 宾语从句之前时，that不宜省略。例如：
Jane realized atonce that she must do with it herself. 珍立刻意识到她必须亲自处理那件事。(如省略that，at once也可能修饰从句，是表示“立刻意识到……”，还是表示“立刻去处理……”，较为含糊)
五、当that宾语从句中的状语部分位于从句的前部时，that不宜省略。例如
Tom promisedthat if he could earn as much as 500 dollars in this month, hewould buy a computer for his brother. 汤姆许下诺言，如果他在这个月里能够赚到500美元，他就给他的弟弟买台电脑。
六、当that宾语从句是双宾语中的直接宾语时，that一般不宜省略。例如：
I will never tellanyone that you have ever been there. 我绝对不会告诉别人你曾经去过那儿。
七、当为了强调而把that引导的主语从句位于句首时，that不可省略。例如：
That they would take the risk was very clear.他们将要冒这个险是很清楚的了。
八、当主句中的谓语动词是固定词组时，其宾语that 从句中的 that不宜省略。例如：
Mary has made up hermind that she will study hard for the scholarship, so she can havethe chance to go abroad for further study. 玛丽下定决心努力学习以获得奖学金资格，这样她就有机会出国深造了。
九、当that宾语从句前有 it作其形式宾语时，that不可省略。例如：
We think itimportant that college students should master at least one foreignlanguage.我们认为大学生至少掌握一门外语是很重要的。
十、当引导同位语从句时，that一般不可省略，尤其当that与先行词处于分割情况时。例如：
He heard thenews that his team had won. 他听到了他的队获胜的消息。
The policereport that the drugs had been found appeared in the pressyesterday. 毒品已经被发现这一警察报告昨天见报了。
There is a feeling inme that they are cheating me. 我有一种感觉他们在骗我。
十一、当that宾语从句的主语是非谓语动词或是主语从句时，that不可省略。例如：
The doctor saidthat taking exercise was the best way of losing weight.医生说运动是最好的减肥方式。
The old lady thenexplained that what she was looking for was a pair of gloves for agirl. 老太太然后说她寻找的是一双女孩子戴的手套。
注意：当名词性从句已有疑问词引导时，不可再加that。例如：
I don't understandthat what you said just now. (×)
I don't understandwhat you said just now. (√)
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