3.that用法真有趣， 两个地方它不去；
逗号后边它不去， 介词之后不考虑；
That做为关系代词，不可用在非限制性定语从句中，即不可用在逗号后边；定语从句中介词之后也不可使用。在逗号或介词后，指物时用which,指人时用who或whom。
例句1）They arrived at a farmhouse, in front of ______sat a small boy.
A. whom B. who C. which D. that
解析：在介词of之后指物，选择C which。
例句 2）The engineer with _____my father works is about 50 years old.
A. whose B. who C. which D. whom
解析：选择 D whom。介词后指人。
例句 3）The letter is from my sister, ______is working in Beijing.
A. which B. that C. whom D. who
解析：选择 D who。逗号后指人，在定语从句中做主语。
4.That which代表物，区别听我来叙述；
先行若是不定代，that就把which 踹；
当先行词为anything，everything，nothing，all，any，much，many，one等不定代词时，只能使用“that”, 不用 “which”。
例：Is there anything that I can do for you? 我能给你做点什么吗？
先行词前有两数， 就用that定无误；
当先行词是基数词或序数词修饰时，只能使用“that”，不用 “which”。
例句： That is the second time that I have been to Japan. 那是我去日本的第二次。
The two buildings that lay on the River Thames fell down last week.
坐落在泰晤士河岸边的那两座大楼上星期倒塌了。
先行词前最高级， 还用that必无疑；
当先行词是形容词最高级或被形容词最高级修饰时，只能使用“that”, 不用 “which”。
例句：This is the most wonderful time that I have ever had.
这是我度过的最美好的时光。
句中若有there be, that应把which替；
例：There is no evidence that animals possess a mysterious sixth-sense allowing them to predict natural disasters. 说动物拥有神秘的第六感，可以预知自然灾害，这是没有根据的。
先行主中做表语， 避免重复从句里；
例句：1）This is not the hospital that it used to be.
这已经不再是以前那座医院了。
2）Mr Thomson is not the man that he was.
汤姆森先生已经不再是以前的他了。
解析：定语从句的先行词均在主句中做表语。
例句：Which is the course that we are to take ?
我们要学哪门课？
解析：句中已有which,后面不再用which,避免重复用that。
（Just the only very same last, 其后也要用that;）
先行词前有Just 、the only、 very 、same 、last等词，关系代词用that，不用which。
例句：1) The North Pole is the only place that you can see polar bears.
北极是唯一能见到北极熊的地方。
2）This is the same bike that he lost.
这就是他丢了的那辆自行车。
注意区分：
3) This is the same bike as he lost.
这辆自行车和他丢的那辆一样。（但不是）