▲首先要明确的是，说“逗号后面不能用 that 引导的从句”是不严谨的，也是不准确的（在许多情况下甚至时错误的）。如果你的老师说“逗号后面不能用 that 引导的从句”，这有两种可能：一种可能是为了把问题简单化，以便学生在初学阶段易在考试中得分（因为这条“规则”在考试时有时可能有效）；另一种可能，它肯定是针对非限制性定语从句而言的——学生在初学阶段，对非限制性定语从句不易理解，做题往往出错，于是老师就给了学生这条“秘籍”。
▲如果是针对非限制性定从句，说“逗号后面不能用 that 引导的从句”，尽管不严谨（因为有无逗号并不是判断非限制性定语从句的唯一标准），但在很多情况下确实还是有效的——如果你对非限制性定语从句没有从根本上弄懂，这句话说不定在考试时能助你一臂之力。比如以下几题均为近几年的高考题，均涉及非限制性定语从句考点，答案均选关系代词 which，而不选 that，而这些非限制性定语从句前均有一个逗号：
1. China Today attracts a worldwide readership, _______ shows that more and more people all over the world want to learn about China. (2015福建卷)
A. who                B. whom                 C. that                   D. which
2. It is a truly delightful place, _______ looks the same as it must have done 100 years ago with its winding streets and pretty cottages. (2015湖南卷)
A. as                  B. where                  C. that                  D. which
3. Until now, we have raised 50,000 pounds for the poor children, ______ is quite unexpected. (2014四川卷)
A. that                B. which                   C. who                 D. it
4. Mo Yan was awarded the Nobel Prize for Literature in 2012, ______ made one of the Chinese people’s long-held dreams come true. (2013安徽卷)
A. it                    B. that                       C. what                D. which
5. We have launched another man-made satellite, ______ is announced in today’s newspaper. (2013天津卷)
A. that                B. which                   C. who                 D. what
6. When deeply absorbed in work, ______ he often was, he would forget all about eating or sleeping. (2012北京卷)
A. that                B. which                   C. where              D. when
7. By 16:30, ______ was almost closing time, nearly all the paintings had been sold. (2012江西卷)
A. which             B. when                   C. what                 D. that
8. That evening, _______ I will tell you more about later, I ended up working very late. (2012全国卷II)
A. that                 B. which                  C. what                  D. when
9. It is the third time that she has won the race, ______ has surprised us all. (2012陕西卷)
A. that                 B. where                  C. which                D. what
▲如果不是针对非限制性定从句，“逗号后面不能用 that 引导的从句”这条“秘籍”就完全失效了，或者说完全错了！比如下面的句子，that引导的是宾语从句，注意动词与宾语从句之间被一个其他的成分隔开了：
He knew, deep down, that he would have to apologise. 他心里知道应该去道个歉。
The jury concluded, from the evidence, that she was guilty. 陪审团根据证据作出结论，认定她有罪。
He felt, with some justification, that he had been unfairly treated. 他感到自己受到了不公平的对待是有一定理由的。
Do you think, perhaps, that what happened today might lend weight to that criticism? 你觉得今天发生的事情可能会使那种批评加剧吗？
The border closure meant, in effect, that no trade took place between the countries. 边境关闭实际上是意味着两国间不再有贸易往来。
The company denies, in the face of overwhelming evidence, that smoking causes cancer. 即使在大量证据面前，公司依然否认吸烟会引发癌症。
She discovered, to her dismay, that her exam was a whole month earlier than she’d expected.  她郁闷地发现考试比她预料的早了整整一个月。
▲又如下面的句子，that 是主句的主语（注意它前面的条件状语从句）：
If this is true, that must be false. 如果这个是真的，那个必然是假的。
If you hear a car, that will be Roland. 你要是听到汽车声，那可能就是罗兰来了。
If it rains, that will spell trouble for the picnic. 如果下雨，那就意味着给野餐添麻烦。
If we mean to catch the early bus, that means getting up before five. 要想赶早班车，我们就得在5点钟以前起床。
If you want to give money to the church, that is optional. 如果你想捐钱给教堂，那是非强制性的。
If a book is in English, that means slow progress for you. 如果一本书是用英语写的，那就意味着你要读慢些。
If you could tell her, that would save me phoning her. 如果你能告诉她的话，就不用我给她打电话了。
If you wish to make a fool of yourself, that is your affair. 如果你想让自己出丑，那是你自己的事。
If he’s only interested in your looks, that shows how shallow he is. 如果他只是对你的外貌感兴趣，那说明他是多么浅薄。
▲事实上，逗号后面用 that 来引出一个句子是非常普遍的用法，而且情况远比上面归纳的要复杂得多。下面是一些例句：
As far as I can see, that is highly unlikely. 依我看那是极不可能的。
When I say two hours, that includes time for eating. 我说两小时，那是包括了吃饭的时间。
When he said the job was in Paris, that clinched it for me. 他说那份工作在巴黎，这就促使我作出了决定。
When I was dieting, that was when my social life started going down the tube. 我节食的时候，也就切断了一切社交生活。
I want to make it clear, however, that no one is untouchable in this investigation. 但我想说明白一点，这次调查谁都逃不了。
It happened, whether by accident or design, that Steve and I were the last two people to leave. 不知是偶然还是事先安排，史蒂夫和我是最后离开的两个人。
With the loyalty of key military units in question, that could prove an extraordinarily difficult task. 由于无法确定关键军事部队忠诚与否，这项任务也许会变得异常困难。