定语从句中，关系代词"that"的使用有一定的规则，以下是一些只能用"that"的情况：
先行词为不定代词
all, much, anything, something, nothing, everything, little, none等。
先行词被序数词、形容词最高级或the only, the very, the just修饰
He was the only person that joined the army in his village that year. 。
先行词既有人又有物
They talked about the people and the things （that） they remembered in the school. 。
主句是以who或which开头的特殊疑问句
Who is the boy that was here just now？ 。
关系代词在从句中作表语
He is not the man that he used to be. 。
需要注意的是，当先行词被形容词最高级或序数词修饰，且从句中不缺成分时，"that"可以省略。此外，当先行词为"anyone", "one", "ones", "those", "he", "people"等词时，通常用"who"而不是"that"。
总结：
当先行词为不定代词或特定形容词修饰且从句缺成分时，用"that"。
当先行词既有人又有物时，用"that"。
当主句是特殊疑问句且以"who"或"which"开头时，用"that"。
当关系代词在从句中作表语时，用"that"。
建议在实际应用中根据具体的句子结构和语境选择合适的关系代词。