高考英语语法：由“介词 关系代词”引导的定语从句
1. 由“介词 关系代词”引导的定语从句主要用于正式文体，在非正式文体中通常将介词放至句末。如：
This is the man to whom I referred. 我指的就是这个人。
This is the man (whom) I referred to. 我指的就是这个人。
2. 直接用于介词后作宾语的关系代词which不能换成that，直接用于介词后作宾语的关系代词whom不能换成who。但若介词用于句末，则用作宾语的which, whom也可换成that, who，或者省略不同。如：
This is a subject about which we have talked a lot. 这是一个我们讨论了许多的问题。(which不能换成that，也不能省略)
This is a subject which we have talked about a lot. 这是一个我们讨论了许多的问题。(which可以换成that，也可以省略)
3. 关系副词when, where, why根据情况有时可换成“介词 关系代词which”。如：
That is the day when [=on which] he was born. 那就是他出生的日子。
That is the house where [=in which] he lived. 那就是他住过的房子。
That is the reason why [=for which] he must apologize. 那就是他必须道歉的原因。
4. 在很正式的文体中，“介词 关系代词”引导的定语从句可紧缩成“介词 关系代词 不定式”结构。如：
在那儿孩子们有个玩耍的花园。
正：There the children had a garden in which to play. (很正式)
正：There the children had a garden in which they could play. (较正式)
正：There the children had a garden to play in. (较口语化)
注：这类“介词 关系代词 不定式”结构中的介词不能没有，也不能放在句末。如不能说There the children had a garden which to play in.