that用作先行詞時后接定語從句的用法(that which型)

當代詞 that 用作先行詞后接定語從句時，定語從句通常用關系代詞which來引導。如：

We have that which we need. 我們需要的東西都有了。

That which I say and do is my own affair. 我說什么做什么是我自己的事。

The true cost often differs from that which had first been projected.  實際成本經常與最初計劃的有所不同。

What is that which you have just stuffed into the plastic cup? 你剛才塞在塑料杯里的是什么東西?

They say the only real knowledge is that which can be measured. 據說唯一真實的知識是可以檢驗的知識。

但 that which 這一說法相當正式，在現代英語中通常改用 what。如：

That which (=What) many people say is not always true. 很多人說的話并不表示一定是真的。

順便說一句，that 后可接定語從句，但 this 后習慣上不接定語從句。