定語從句是非常重要的一個語法點，明確什麼時候用什麼樣的關係代詞和關係副詞非常重要。一般情況下，當先行詞是人的時候用who和whom，當先行詞是物的時候，用which，而that一般是都可以用，當然也有一些例外，下面是隻能用that引導定語從句的十三種情況。
01句子的主語是先行詞，而且又是物
在there + be 的句型中，句子的主語是先行詞，而且又是物。例如：
There are two novels that I want to read. 我要讀的有兩本小說。
There is no work that can be done now. 沒有什麼工作現在能做的了。
02先行詞為主句的'表語或關係代詞為從句的表語
當先行詞為主句的表語或者關係代詞為從句的表語時。例如：
This is the book that was bought yesterday.這就是昨天買的書。
Our school is no longer the school that itused to be. 我們的學校不再是以前的學校了。
03以Here is (are)開頭的句子時
以Here is (are)開頭的句子時。例如：
Here is a film that will move anyone.這是一部將使任何人受感動的電影。
Here are two books that I will buy.這是我要買的兩本書。
04It is (high)time +定語從句時
It is (high)time +定語從句時。例如：
It is time that we should have a rest.我們應該休息了。
It is high time that they started out. 他們該動身了。
05先行詞是way，關係代詞用that或者in which
當先行詞是way等詞時，關係代詞用that或者in which在定語從句中作方式狀語時，在口語中，常可省略。例如：
This is the way that my father did thiswork.這就是我父親做此工作的方式。
She admired the way in which I answered thequestions. 她羨慕我回答問題的方式。
06雙重限定性定語從句中，如果一個從句用who或者which引導時，另一個從句用that引導。
在雙重限定性定語從句中，如果一個從句用who或者which引導時，那麼另一個從句用that引導。例如：
He is the student that I have ever see whocan jump highest.他是我曾經看到過跳得最高的學生。
My brother studies in the school which isthe most beautiful in our city that isn’t far from here. 我的弟弟在我們的城市最美麗的學校讀書，並且離這兒不遠。
07當先行詞被the last , thevery 和the only修飾時
當先行詞被the last , thevery 和the only修飾時。例如：
This is the very pen that I am looking for.這正是我找的鋼筆。
The only book I want to read is missing. 我唯一想看的書不見了。
08在強調句子中，並且以who ,which,what開頭時
在強調句子中，並且以who ,which,what開頭時。例如：
Who was it that was lost ? 究竟是誰迷路了？（此句中，最好不用who來代替that，避免重複。）
What was it that you did last week? 你上週究竟做什麼了？
09當先行詞有序數詞時
當先行詞有序數詞時。例如：
You are the first person that I want to askfor.你是我要見的第一個人。
This is the second book that I have everwritten.這是我寫的第二本書。
10先行詞被all, every, no ,some, few , 等詞修飾時
當先行詞被all, every, no ,some, few , little, much, both等修飾時，例如：
This is all that I want to say at themeeting.這就是我在會上要說的。
Have you any books that are worth reading? 你有值的看的書嗎？
11當先行詞既指人又指物時
The professor and his achievement that Iheard about are admired by them .我聽說的那位教授和他的成就得到他們的讚美。
Let’s talk about thepersons and the things that we can remember.讓我們談論能夠想起的人和事。
12當先行詞為anything、everything、nothing時
當先行詞為anything、everything、nothing時，關係代詞用that,不用which；但用something時，用which或者that均可。例如：
Everything we have seen in China is moving. 我們在中國看見的東西件件感人。
I have nothing that is worth reading. 我沒有什麼值得一讀的東西。
13當先行詞是疑問代詞who時
當先行詞是疑問代詞who時。例如：
Who that you have ever seen can beat him inchess? 你曾見過誰能在棋藝上打敗他？
