1. 先行词是all , much, few, little, everything, anything, nothing 等不定代词时。
eg. Is there anything that you want to eat?
2. 先行词被all, few, little, much, every, some, no等词修饰或被the only, the very, the same, the last 等限定词修饰时。
eg. These are all the pictures that I have seen.
There are some apples that I bought this morning.
eg. My father and his teacher talked a lot about the things and the persons that they could remember.
eg. Our city is not the one that it used to be.
eg. This is the best place that I have ever been to.
6. 主句是who 或which 引导的特殊问句时。
eg. Who is the boy that wearing a red hat?
Which is the book that we need to read this week?