一、当不定代词作先行词的时候，或者用all，any，little，few，much，some，every，no来修饰先行词的时候，定语从句的关系词只能使用that。我们经常使用的不定代词有all，some，everything，nothing，none，few，anything等等。 比如：
There is all that I can do for you.这是我能为你做的所有事。
二、当有序数词修饰先行词的时候，定语从句的关系词只能使用that。常用的序数词有：first，second等等。 比如：
The first place that they arrived was Beijing.他们到达的第一个地方是北京。
三、当有形容词最高级修饰先行词的时候，定语从句的关系词只能使用that。 比如：
This is the best book that I have looked.这是我看过最好的书。
四、当the only，the last，the very修饰先行词的时候，定语从句的关系词只能使用that。 比如：
He is the only boy that has looked this book. 他是唯一一个读完这本书的男孩子。
五、当疑问代词who，which等出现在先行词前面的时候，定语从句的关系词只能使用that。 比如：Who is the girl that wear red dress? 穿红色裙子的女孩是谁？
六、当人和物同时做先行词的时候，定语从句的关系词只能使用that。 比如：
The singer and her song that you told me are very nice. 你告诉我的歌手和她的歌曲都非常棒！
七、当way，reason来作先行词的时候，关系代词经常是用that。 比如：
It is the only way that you can finish the work. 这是你完成工作的唯一方法。
定语从句用that的举例
We should do all that is useful to the people. 我们应该做一切有益于人民的事情。
Is there anything that I can do for you?有什么需要我帮忙的吗？
There’s nothing that can be said about it关于这件事，已没有什么可说的了。
Do you mean the one that you bought yesterday?你指的是昨天买的那个吗？
I have finished reading all the books that I borrowed from the library. 我己经看完了所有从图书馆借来的书。
He has learned many English new words that the teacher taught him last week by heart.上周老师教的许多新英语单词，他都用心记住了。
The only thing that we can do was to wait. 我们唯一能做的事情就是等待。
This is the first book that I bought in the shop . 这是我在这家书店买的第一本书。
This is one of the most exciting football games that I have ever watched 这是我看过的最激动人心的足球赛之一。
This is the last film that has been shown in our city this year.这是我市今年放映最后一部影片。