题目内容：

He is the best student that I have seen这句的先行词是the best student 然后因为有the best最高级修饰
所以用that 这样理解可以吗?

优质解答

这样理解完全正确.

定语从句中关系代词用that（不用which）的情况：
1. 先行词是all, anything, everything, nothing等,关系代词应用that
That's all that he told me.
2.先行词前有the only, the same, the very 或no, little, much, any, every, all 等修饰时,关系代词应用that
She is the only person that I know in this company.
3. 先行词前有序数词或形容词最高级修饰时,关系代词常用that
The first thing that I want to do is to take it away.
4. 当先行词同时含有"人"或"物"时,关系代词应用that
5. 在固定结构the same...that...; so...that...; such...that...以及it is/was...that...的强调句型中,要用that
This is the same park that we once visited.
6. 如主句以there be开头时,关系代词应用that
There is a room that has two windows.

希望能帮到你,祝你开心.