非限制性定语从句
起补充说明作用，去掉不影响主句意思；用逗号隔开，且不用 that。
…, which … / …, who …
要点
- 用逗号与主句隔开，去掉之后主句意思仍然完整。
- 不用 that 引导，指人用 who / whom，指物用 which。
- which 可以指代整个主句所说的内容。
- 关系词不能省略。
- as 也能顶住整句，且能放在句首：As we had expected, he won the game. —— 与 which 的分别是 as 带"正如 / 正像"这层意思，位置也比 which 活（which 那一句只能跟在后面）。
- 逗号这一头并不改关系词的选择，只改掉"能不能用 that"：Beijing, where I grew up, is freezing in winter. ／ My mother, whom I called last night, is staying with us. —— 关系副词与 whom 照样能进非限制性。
- 非限制性从句能把两三句短话叠成一句：My sister, who lives in Guangzhou, sends me fruit every month. —— 结构上整句仍只有一套主谓，判句子类别时按复合句算（见「简单句·并列句·复合句」）。
例句
My brother, who lives in Beijing, is a doctor.
我哥哥住在北京，他是一名医生。
He passed the exam, which made his parents happy.
他通过了考试，这让他父母很高兴。
The book, which I bought yesterday, is very useful.
这本书我昨天买的，很有用。
She came late again, which surprised us all.
她又迟到了，这让我们都很意外。
As we had expected, he won the game.
和我们预计的一样，他赢了这场。
Beijing, where I grew up, is freezing in winter.
我是在北京长大的，那儿冬天冷得刺骨。
易错提示
- 有逗号就是非限制性，一律不能用 that。
- which 指代整个主句时，谓语用单数。
- 逗号一加，关系词就不能省：My brother who lives in Beijing is a doctor 少了那对逗号，意思就变成"好几个兄弟里住北京的那一个" —— 逗号在这里改的是句意，不只是停顿。
- which 顶整句时谓语用单数（老规矩已点）；as 顶整句时，动词跟着 as 后面那部分走：As is known to all, … —— 两式别当成同一格。