关系代名词可以搭配适当的介系词，用来代替副词片语，这时不要忘了加上该加的介系词：
This is the formula [by which I solved the math problem ______ ].→方式
I used a sharp knife [with which I opened the can _____ ].→工具
John is the guy [with whom I went to the party _____ ].→同伴
I went to a park [in which I found the stray dog ____ ].→地点
这些关系代名词，因为出现在介系词后面，所以不能省略。搭配介系词的结果就等于疑问副词：
in which →where →I went to a park where I saw the dog.
by which →how →This is the formula how problems can be solved.
——劉美君. (2012). 英文文法有道理! 重新認識英文文法觀念. 聯經.
