英语中什么情况下使用被动语态

第一是当我们不知道谁是动作的执行者时，只能用被动语态。如：70% of the surface of the earth is covered by water. （百分之
七十的地球表面被水覆盖着。）
第二，当我们要强调动作的承受者时要用被动语态。如：He was praised yesterday. (他昨天受到表扬了。）

1. 不知道动作行为人是谁
   

   例：My office was broken into when I was on holiday. （不知道闯入者的身份）

2. 行为人的范围很广

   例：An order from can be found on page 2. （指任何填表的人）

3. 行为人是谁不重要

   例：He is thought to be somewhere in Russia. （谁认为的不重要）

4. 行为人的身份非常明显

   She is being treated in hospital. （很明显病人被医护人员治疗）

5. 描述客观现象的过程时通常忽略动作行为人的身份，只强调被执行的对象及执行过程。

   例：Nuclear waste will still be radioactive even after 20,000 years, so it must be disposed of very carefully. I t can be stored as a liquid in stainless-stell containers which are encased in concrete. （处理核污染的过程比谁来处理更重要）

6. 在口语对话中，当说话人不知道行为人身份，通常会用people, somebody/someone, something, they, we或you来指代，在正式书写中，则用被动彻底避开提到行为人身份。

   例：They're installing the new computer system next month. （口语）

   The new computer system is being installed next month. （书面）

   甚至可以用动词衍生名词作为主语来表达相同的意思

   例：The installation of the new computer system will be completed by next month.

7. 延续上一句话谈论的对象时用被动

   例：The three machines tested for the report contained different types of safety valve. All the valves were manufactured by the Boron Group in Germany. （第一句已经谈到了valve，后续继续谈用被动）

8. 当主语很长的时候，用被动可以让句子更自然

   例：I was surprised by Dev's decision to give up his job and move to Sydney. （主动句Dev's decision to give up his job and move to Sydney surprised me虽然语法不错，但是主语‘Dev's decision to give up his job and move to Sydney ’很长，显得句子很拗）

9. it引导的被动从句比that引导的被动从句更自然

   例：主动句：Everybody believed (that) the plan would fail. 

   被动句：It was believed that the plan would fail. （自然）

   That the plan would fail was believed by everybody. （不自然）

不知道动作行为人是谁
例：My office was broken into when I was on holiday. （不知道闯入者的身份）
2.
行为人的范围很广
例：An order from can be found on page 2. （指任何填表的人）
3.
行为人是谁不重要
例：He is thought to be somewhere in Russia. （谁认为的不重要）
4.
行为人的身份非常明显
She is being treated in hospital. （很明显病人被医护人员治疗）
5.
描述客观现象的过程时通常忽略动作行为人的身份，只强调被执行的对象及执行过程。
例：Nuclear waste will still be radioactive even after 20,000 years, so it must be disposed of very carefully. I t can be stored as a liquid in stainless-stell containers which are encased in concrete. （处理核污染的过程比谁来处理更重要）
6.
在口语对话中，当说话人不知道行为人身份，通常会用people, somebody/someone, something, they, we或you来指代，在正式书写中，则用被动彻底避开提到行为人身份。
例：They're installing the new computer system next month. （口语）
The new computer system is being installed next month. （书面）
甚至可以用动词衍生名词作为主语来表达相同的意思
例：The installation of the new computer system will be completed by next month.
7.
延续上一句话谈论的对象时用被动
例：The three machines tested for the report contained different types of safety valve. All the valves were manufactured by the Boron Group in Germany. （第一句已经谈到了valve，后续继续谈用被动）
8.
当主语很长的时候，用被动可以让句子更自然
例：I was surprised by Dev's decision to give up his job and move to Sydney. （主动句Dev's decision to give up his job and move to Sydney surprised me虽然语法不错，但是主语‘Dev's decision to give up his job and move to Sydney ’很长，显得句子很拗）
9.
it引导的被动从句比that引导的被动从句更自然
例：主动句：Everybody believed (that) the plan would fail.
被动句：It was believed that the plan would fail. （自然）
That the plan would fail was believed by everybody. （不自然）