定语从句在什么情况下只能用that

1、定语从句修饰的词（即，先行词），前面有形容词最高级时。

This is the best film that I have seen.

先行词是film，前面有the best修饰它，这时候只能用that，强调“最好的”。

2、先行词被the very/the only/the same/the last等修饰时。

She is the only girl who has got a prize.

先行词是girl，前面有the only修饰它，这时候只能用that，强调“唯一一个”。

3、先行词被序数词修饰时。

The first place that they visited was the Big Ben.

先行词是place，前面有the first修饰它，这时候只能用that，强调“第一个”。

定语从句关系构成

关系从句的句法功能主要是充当定语。在英语中，关系从句通常位于它所修饰的词（组）之后。被关系从句修饰的词（组）叫做先行词，引导关系从句的词称为关系词，关系词指代先行词并在关系从句中充当一定的成分。例如：

This is the book which interests me.

“which interests me”是关系从句，修饰先行词“book”，而关系词“which”指代先行词“book”并在关系子句中充当主语。

这句话可以拆分为两个句子来理解：“This is the book”和“The book interests me”。

很高兴为您解答：
只能使用that,不用which 的情况：
1.只能使用that,不用which 的情况：
(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.
2.只能用which,不用that 的情况：
(1) 在非限制性定语从中.例如：
The meeting was put off,which was exactly what we wanted.
(2) 定语从句由介词+关系代词引导,先行词是物时.例如：
The thing about which he is talking is of great importance.