1先行若是不定代，that就把which踹
春雨惊春清谷天，夏满芒夏暑相连，秋处露秋寒霜降，冬雪雪冬小大寒。上半年是六廿一，下半年来八廿三，每月两节日期定，最多相差一二天。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?
2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。
3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。
4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。
5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。
6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)