定语从句只能用that的情况包括：
先行词为不定代词
all, everything, anything, nothing, little, much, some, any, few等。
例如：Is there anything that I can do for you？ ；I don't have anything that is yours.
先行词被不定代词修饰
all, any, few, little, much, no等。
例如：I have finished all my homework that our teacher distributed yesterday. ；There is little water that is clean.
先行词为序数词或被序数词修饰
first, second, third, etc. 。
例如：I'm the first student that passed the exam. ；The first English book that I read was "The Prince and the Pauper" by Mark Twain.
先行词被形容词最高级修饰
the best, the worst, the most interesting, the least expensive, etc. 。
例如：This is the best film that I have ever seen. ；This is the most wonderful time that I have ever had.
先行词既有人又有物
在定语从句中，当先行词既有人又有物时，关系词只能用that。
例如：Do you know the things and persons that they are talking about？
主语是以who/which开头的特殊疑问句
在定语从句中，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 person that called you？
先行词被the only, the very, the just等限定词修饰
the only, the very, the just, the last等。
例如：He is the very man that helped the girl out of the water.
这些情况是定语从句中关系词只能使用that的典型例子。需要注意的是，在特定情况下，关系代词who和which也可以用于定语从句，但上述情况中只能使用that。