被动语态用法：
1)当我们不知道动作的执行者是谁，或者没有必要指出动作的执行者时，需用被动语态。
2)当我们需要强调动作的承受者时，常用被动语态。
3)如果需要说出动作的执行者， 用by引导出动作的执行者。
主动语态变为被动语态时，其谓语动词的时态要与原句时态保持一致，其谓语动词的数要与新主语保持一致。
主动语态变为被动语态时有以下几种情况：
1)主语+谓语动词+宾语
将主动语态的宾语变为被动语态的主语。
(主动)We bought a book yesterday.
(被动)The book was bought yesterday.
2)主语+谓语动词+间接宾语+直接宾语
将主动语态中一个宾语变为被动语态的主语。多数情况下将间接宾语变为主语。如果直接宾语变为主语时，间接宾语前要加介词to / for.
(主动)He showed me a book yesterday.
(被动)I was showed a book yesterday.
(被动)The book was showed to me yesterday.
3)主语+谓语动词+复合宾语
含有一个由宾语加宾语补足语构成的复合宾语，变为被动语态时，将主动语态的宾语变为被动语态的主语，宾语补足语保留不变，成为主语补足语。
(主动)I found him a good pupil. (宾语补足语)
(被动)He was found a good pupil. (主语补足语)
4)短语动词变为被动语态
有些短语动词相当于一个及物动词，其后可以接宾语，因此它们也有被动语态，但短语动词是不可分割的整体，变为被动语态时不可去掉其后面的介词或副词。
(主动)We should look after the patients very well.
(被动)The patients should be looked after very well by us.
5)宾语从句变为被动语态
若主动语态中是宾语从句，变为被动语态时常用it作形式主语，而把真正的主语放在后面。
Its said that he passed the exam.
被动语态应注意的几个特殊问题：
(1)不及物动词带介词和介词宾语，在变为被动语态时，仍然要带上介词。
(主动)The students in class listen to the teacher carefully.
(被动)The teacher is listened to carefully by the students in class.
(2)当动词带有复合宾语时，并且宾补是省去to的动词不定式时，在被动语态中应加上to。
(主动)They make do all the work.
(被动)We were made to do all the work.
(主动)We often hear her sing English songs.
(被动)She is often heard to sing English songs.
(主动)I see him walk to school.
(被动)He is seen to walk to school.