定语从句在句中作定语，修饰某一名词或代词。从句所修饰的词叫先行词，定语从句必定是要放在先行词之后的。定语从句由关系代词或副词引导。其中关系代词who，whom，whose，which和that。关系副词 when， why， where等。例如：
1. We went to the park which / that we often visited.
我们去了那个常去的公园。
2. I can never forget the day when I began my studies in Beijing University.
我绝不会忘记我在北大开始学习的那一天。
3. The ladder on which I was standing began to slip.
我站的那个梯子开始滑落。
4. The wine, which was in the cellar, was all ruined.
放在地下室的葡萄酒都被毁了。