定语从句主要分为两类：限制性定语从句和非限制性定语从句。 限制性定语从句和主句关系密切，它和主句之间不可用逗号隔开，且一般不可以省去。 非限制性定语从句是对先行词的附加说明，它和主句之间往往用逗号隔开。 限制性定语从句。例如： ① This is the best filmthat I have ever seen. 这是我看过的最好的电影。 ② She is the womanwho telephoned the police. 她就是给警察打电话的女士。

非限制性定语从句:which/who/when/where/whose/whom为引导词的非限制性定语从句。例如： ① He came from Beijing, which is the capital of China. 他来自于北京——中国的首都。 ② I meet your mother, who is wearing a red dress. 我碰到了你妈妈，她穿着红色连衣裙。

③ I am invited to a party, which makes me so happy. 有人邀请我参加聚会，这让我很高兴。 ④ The car accident happened in 1990, when I was a six-yearold child. 这场车祸发生在1990年，那一年我六岁。⑤ They went to London, where they lived for six months. 他们去了伦敦，在那里住了六个月。 ⑥ The boy, whose father is a teacher, studies very hard. 这个男孩学习很努力，他的爸爸是老师。 ⑦ Peter, whom you met in Italy,is now back in China. 彼得现在回中国了，你之前在意大利见过他。 (2)as为引导词的非限制性定语从句。例如： ① As is so often pointed out, knowledge is a two-edgedweapon which can be used equally for good or evil. 正如人们常说的那样，知识是把双刃剑，既可以用来造福人类，也可以危害人类。

② Stratford-on-Avon, as we all know, has only one industry—William Shakespeare—but there are two distinctly separate and increasingly hostile branches. 众所周知，埃文河畔斯特拉特福镇只有一个产业，即威廉·莎士比亚，但是存在两个明显不同且日益敌对的分支。 ③ He forgot to bring his pen with him, as was often the case. 他常常忘记带笔，这次又是如此。 注as引导的非限制性定语从句有两个特点。一：as指代的先行词通常是一个完整的句子;二：as引导的定语从句位置非常灵活，可以在句首、句中或句末。