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：这是一个我们讨论了许多的问题。This is a subject about which we have talked a lot.(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.