1. 当先行词是不定代词或被不定代词修饰时.如 Tell me everything that you know. / All the people that are working are my workmates.
2. 当先行词是序数词或被序数词修饰时.如 I want the first that looks bigger. / This is the third dictionary that I have used.
3. 当先行词是最高级或被最高级修饰时.如 Tom is the best that I have ever taught. / The most important thing that she can do is to go to bed at once.
4. 当先行词是the only (“唯一的”),the very (“恰恰；就”),the just (“恰恰；就”),the same (“相同的”),the last (“最后一个”)等或被这些词修饰时.如 The only thing that I need to do is to cook a meal. / He’s the very person that I want to find. / Did you miss the last problem that is in your math test?
5. 当先行词是数词时.如 You can catch the four that are dead.
6. 当先行词作表语时.如 My hometown is no longer the place that it used to be.
7. 当先行词中既有人,又有物时.The young man and his bike that I saw just now are still there.
8. 当句子中含有两个定语从句时,如果其中一个定语从句的引导词用了which、who、whom,则另一个定语从句的引导词用that,以避免重复. 如 The TV set which I ught from the shop that opened last month was broken. / The man who is standing behind the people that are playing cards is my uncle.
9. 当which、who、whom作疑问词,引导特殊疑问句时.如 Which took the camera that was made in Japan? / Who knows the y that lives upstairs?