(1) 先行词是all,few,little,nothing,everything,anything 等不定代词时.例如：
All that he said is true.
(2) 先行词被only,no,any,all,等词修饰时.例如：
He is the only foreigner that has been to that place.
(3) 先行词是序数词或被序数词修饰的词.例如：
He was the second (person) that told me the secret.
(4) 先行词是形容词最高级或被形容词最高级修饰的词.
This is the best book (that) I have read this year.
(5) 先行词既包括人又包括物时.例如：
He talked about the people and the things he remembered.