一、一般被动语态作后置定语时be 动词可以省略。
比如说：The story wrote by him was so exciting. wrote by是后置定语，前面就不用加be 动词，如果一定要加，就要加“that is”（变成定语从句）
二、当从句中主语与主句中主语一致,而且从句中含有系动词时,可以省略从句中的主语和系动词.
When asked to clean the room, he was unhappy.
When (he was) asked to clean the room, he was unhappy.
三、如果过去分词做状语时也可以省略be
如: Cheated by the old man, the girl was very angry.
扩展资料：
被动语态例句
A person who is truly honest is called a straight arrow.
直言不讳的人才是真正诚实的人。
A note was passed up to the speaker.
有人给讲演者递上来一张纸条。
John was elected president of the class instead of Harry.
乔治被选为班长而代替了亨利。
Volcanoes are described as active, dormant or extinct.
火山被描述为活的，沉睡着的，或者死的。
The soldier was killed, but the train was saved.
这位战士牺牲了，然而列车得救了。
He was thought to be clever but dishonest.
他被认为很聪明但不诚实。
The first zoological garden in the United States was established in 1874.
美国的第一个动物园是1874年建立的。
被动语态的be在一般情况下都是不可以省略的。但是有两种情况可以。
1.前置定语修饰的时候
The broken glass破碎的杯子
2.后置的定语从句中省略be动词
The man() given a praise is my uncle.被表扬的那个男人是我的叔叔
省略的部分是“Which is be”。
被动语态
被动语态(passive voice)是动词的一种形式，用以说明主语与谓语动词之间的关系。英语的语态共有两种：主动语态和被动语态。主动语态表示主语是动作的执行者，被动语态表示主语是动作的承受者。被动语态是动词的一种特殊形式，一般来说，只有需要动作对象的及物动词才有被动语态。汉语往往用"被"、"受"、"给"等被动词来表示被动意义 。被动语态由“助动词be+及物动词的过去分词”构成。被动语态的时态变化只改变be的形式，过去分词部分不变。疑问式和否定式的变化也如此。
构成
（1）助动词be+及物动词的过去分词、
（2）情态动词+be+及物动词的过去分词
特殊用法
1.不及物动词无被动语态。 如：happen, break down, come out......
What will happen in 100 years.
The dinosaurs disappeared about 65 million years ago.
2.有些动词用主动形式表示被动意义。 如： write, sell, ride.....
This pen writes well.
This new book sells well.
3.感官动词或使役动词使用省略to的动词不定式，主动语态中不带“to”，但变为被动语态时，须加上“to”。
例：make somebody do something→somebody+ be +made to do something
see somebody do something→somebody +be +seen to do something
A girl saw my wallet drop when she passed by.→My wallet was seen to drop by a girl when she passed by.
The boss made the little boy do heavy work.→The little boy was made to do heavy work by the boss.
4.如果是接双宾语的动词改为被动语态时，直接宾语（物）作主语，那么动词后要用介词，这个介词是由与其搭配的动词决定。
He gave me a book.→A book was given to me by him.
He showed me a ticket.→A ticket was shown to me by him.
My father bought me a new bike. →A new bike was bought for me by my father.
5.一些动词短语用于被动语态时，动词短语应当看作一个整体，而不能丢掉其中的介词或副词。
We can’t laugh at him. →He can’t be laughed at by us.
He listens to the radio every day. →The radio is listened to by him every day.
The nurse is taking care of the sick man. →The sick man is being taken care of by the nurse.
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