1.当先行词是all /much /little/few /something /anything /everything /nothing/none等不定代词的时候，在定语从句中，先行词只能用that.
例如：Is there anything that I can do for you?有我可以为你做的事情吗？
2.当先行词前面有all/no/little/few/any等修饰时，只能用关系代词that。
例如：I have finished all my homework that our teacher distributed yesterday.我已经完成了所有老师昨天发下来的家庭作业。
3.当先行词被the/only/the last/the very修饰的时候，定语从句只能用that。
例如：Eating is the only thing that interest me.吃是唯一一件使我感兴趣的事情。
4.当先行词是序数词或被序数词修饰时，关系词只能用that。
例如：I'm the first student that passed the exam.我是第一个通过考试的人。
5.当先行词被最高级修饰时，只能用that做关系代词。
例如：The tallest boy that sits in front of you is my friend.坐在你前面那个最高的男生是我的朋友。
6.当先行词既有人又有物时，只能用关系代词that。
例如：Our teacher told us many interesting things and persons that he met during traveling.我们老师告诉我们很多在他旅行期间遇到的有趣的事和人。
7.当主语是以who/which开头的特殊疑问句时，关系代词只能用that。
例如：Who is the girl that is singing in the front of the classroom.正在教室前唱歌的女孩是谁？
