在定语从句中，"that"作为关系代词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
anything, everything, nothing, all, any, much, few, little等。
例句：Is there anything that I can do for you？
先行词被序数词修饰
first, second, third, ...等序数词。
例句：He was the first person that passed the exam.
先行词被形容词最高级修饰
the most interesting, the tallest, the best等。
例句：This is the most interesting film that I have ever seen.
先行词被序数词或最高级修饰，并且先行词本身也是序数词或最高级
the very first, the very last, the only等。
例句：She is the only person that I trust.
主句是以who或which开头的特殊疑问句
who, which作为主语时，定语从句通常用that引导。
例句：Who is the boy that was here just now？
先行词既有人又有物时
在定语从句中，如果先行词同时包含人和物，通常用that引导。
例句：They talked about the people and the things （that） they remembered in the school.
先行词被the very, the only, the first, the last等词修饰时
这些词修饰先行词时，定语从句通常用that。
例句：Eating is the only thing that interests me.
需要注意的是，在特定情况下，关系代词"which"也可以用于定语从句，但上述规则适用于"that"作为唯一关系代词的情况。希望这些信息对你有所帮助。