关于"that which"的连用，综合权威资料整理如下：

一、基本用法

结构特点

- "that which"在定语从句中可表示"that is/are"，用于简化句子结构，避免重复。

- 例如：

- The letter that whichI received was very important.

- 简化后：The letter I received was very important.

功能解析

- that：

作为关系代词，替代先行词在从句中作主语或宾语。

- which：作为定语，修饰先行词，表示所属关系。

二、使用规则

可互换性

- 在定语从句中，that和which可互换使用，但需根据语境选择：

- that：

用于限制性定语从句，先行词被先行词本身或上下文明确限定；

- which：用于非限制性定语从句，先行词需被逗号隔开。

不宜用which的情况

- 当先行词为不定代词（如all, any, everything等）时，必须用that；

- 先行词被形容词最高级、序数词、very、only等修饰时，也需用that。

三、典型例句

that的用法

- The book thatI borrowed from the library is fascinating.

- The company thatlaunched the new product last year has grown rapidly.

which的用法

- The movie whichwon the Oscar was directed by a famous director.

- The problem whichwe encountered was unexpected.

四、注意事项

早期文学作品中常见"that which"结构，但现代英语中更推荐使用简化形式（如that I received）；

省略规则：

当关系代词在从句中作宾语时，可省略（如：

The letter which I received was important → The letter I received was important. 。

通过以上规则和示例，可灵活掌握"that which"在定语从句中的用法。