定语从句的引导词分为关系代词（who, whom, whose, which, that）和关系副词（where, when, why）两类，这些引导词在引导定语从句的同时均在从句中充当成分。关系代词可作主语、宾语和定语，作宾语时可省略；关系副词可作状语，一般不能省略。下面简单介绍一下定语从句的难点。
一、介词（短语） + 关系代词 / 副词
1. 此处关系代词只能是which / whom / whose / where / when，而不能用who / that。如：
They arrived at an old building, in front of which stood a tower.
He climbed up to the top of the tree, from where he could see clearly what was happening in the woods.
