Question

定语从句中，先行词为the one 时，关系代词是不是只能用that

Answer 1

如果先行词the one在句子（主句或从句都行）中作表语，则该问题为肯定答案。

定语从句中只用that，不用which的情况：

1. 先行词是all,much,little,few,something, anything,everything,none,onthing 等不定代词。

2. 先行词是to be 后面的表语，或者关系代词本身在从句中作表语。

   例如 The school is quite different from the one that it used to be. 

3. 主句以here, there开头。

   例如 Here is the DVD player that you have been looking for.

4. 先行词有the only,the same, the last 等副词修饰。

5. 先行词是序数词，形容词最高级或被序数词或形容词最高级修饰。

6. 主句是以who, which 开头的特殊疑问句

   例如 Who is the person that is standing at the gate?

7. 先行词是两个或两个以上，分别表示人和物

   例如 I am going to talk about the countries and people that I have visited.

Answer 2

也可用 who 或 whom
That's the one who I will marry to. 那便是我要嫁给他的那个男人
She was the one/girl whom you met at the airport. 她就是你在机场遇见的那个女孩。

Answer 3

the one, the only 都是