什么时候用被动语态？主动变被动的三步转换，中考常考
引言
知道被动语态的结构还不够，考试还会考：什么时候用被动？主动句怎么变被动？这篇文章把主动变被动的三步转换和常见考法讲清楚。
什么时候用被动语态
情况一：不知道动作是谁做的
```text
My bike was stolen yesterday.（我的自行车昨天被偷了。）
```
不知道是谁偷的，用被动。
情况二：强调动作的承受者
```text
The bridge was built in 1990.（这座桥建于1990年。）
```
重点在桥，不在建桥的人。
情况三：动作发出者不重要或不言自明
```text
English is taught in middle schools.（初中教英语。）
```
不用说"老师教英语"，重点在英语被教。
主动变被动三步法
第一步：把主动句的宾语变成被动句的主语
```text
主动：Tom broke the window.
被动：The window was broken...
```
第二步：把动词变成 be + 过去分词
```text
broke → was broken
```
be 动词的时态和原句一致，数和新主语一致。
第三步：把主动句的主语放在 by 后面
```text
主动：Tom broke the window.
被动：The window was broken by Tom.
```
by Tom 可以省略，尤其是当动作发出者不重要时。
实战转换
```text
主动：He cleans the room every day.
被动：The room is cleaned by him every day.
主动：She wrote a letter yesterday.
被动：A letter was written by her yesterday.
主动：We will finish the work tomorrow.
被动：The work will be finished tomorrow.
主动：They have built a new school.
被动：A new school has been built.
```
中考常考易错点
易错点一：be 动词的数要和新主语一致
```text
错：The windows was broken.
对：The windows were broken.（windows 是复数 → were）
```
易错点二：不及物动词没有被动
happen / take place / appear / disappear 等不及物动词，没有被动形式。
```text
错：The accident was happened yesterday.
对：The accident happened yesterday.（事故发生了。）
```
易错点三：短语动词的介词不能丢
```text
The children are well looked after.（孩子们被照顾得很好。）
look after 是短语动词，after 不能丢。
```
总结
主动变被动三步：
```text
- 宾语 → 主语
- 动词 → be + done（时态和数要一致）
- 主语 → by + 宾语（可省略）
```
```text
今日小抄：
主动：Tom broke the window.
被动：The window was broken by Tom.
happen 没有被动
look after 被动 → be looked after
```