被动语态怎么构成,被动语态的构成，是be+done吗
网友回答
Your letter must have been lost in the post office( by them ).
首先主动该被动，主动句中的宾语变成被动句主语，即the letter.
主动句主语作为动作的发出者，在被动句中是类似于宾语的存在，by+主语的宾格放在句末。因为被动语态常常用在说话者不知道动作发出者（Someone,Somebody这类的不定代词）或者不想提及动作发出者身份时使用，故也可以省略。
然后谓语根据时态改为被动语态，基本上就可以认为是由原时态主动形式+be的对应形式+谓语动词的过去分词（不是过去式）构成。
例如：is doing->is being done
这里则是must have done->must have been done
想必已经做了-》想必已经被做了
基本上就是这样的。
网友回答
主动语态改被动语态的方法
1）将主动语态改为被动语态应注意以下三个方面：
①将主动语态的宾语改为被动语态的主语；
②将主动语态的谓语动词改为“be＋过去分词”结构；
③将主动语态的主语改为介词by之后的宾语，放在谓语动词之后（有时可省略）。
2）含直接宾语和间接宾语的主动语态改为被动语态时有两种情况：
①把间接宾语改为被动语态的主语，直接宾语仍保留原位；
②把直接宾语改为主动语态的主语，此时，间接宾语前要加介词to或 for。如： He gave the boy an apple．→The boy was given an apple．（或An apple was given to the boy．） Her father bought her a present．→She was bought a present by her father．（或A present was bought for her by her father．）
3）不带to的动词不定式作宾语补足语的主动语态，改为被动语态时不定式前要加to。如： They watched the children sing that morning．→The children were watched to sing that morning．
4）带复合宾语的动词在改为被动语态时，一般把主动语态的宾语改为主语，宾语补足语在被动语态中作主语补足语。如： We call him Xiao Wang．→He is called Xiao Wang． He cut his hair short．→His hair was cut short． They told him to help me．→He was told to help me．
5）短语动词是不可分割的整体，改为被动语态时要保持其完整性，介词或副词不可遗漏。如： We must take good care of the young trees．→The young trees must be taken good care of．
6）含有宾语从句的主动结构变为被动结构时，通常用it作为被动结构的先行主语，从句放在句子后面；也可采用另一种形式。可以这样转换为被动结构的动词有know，say，believe，find，think，report等。如： People believe that he is ill．→It is believed that he is ill．（或：He is believed to be ill．）
1. 一般现在时的被动语态构成：is / am / are + 及物动词的过去分词
Our classroom is cleaned everyday.
I am asked to study hard.
Knives are used for cutting things.
2. 一般过去时的被动语态构成：was / were + 及物动词的过去分词
A new shop was built last year.
Dinosaur eggs were laid long long ago.
3. 现在完成时的被动语态构成：has / have + been + 及物动词的过去分词
This book has been translated into many languages.
Many man-made satellites have been sent up into space by many countries.
4. 一般将来时的被动语态构成：will+ be + 及物动词的过去分词
A new hospital will be built in our city.
Many more trees will be planted next year.
5. 含有情态动词的被动语态构成：情态动词+ be + 及物动词的过去分词
Young trees must be watered often.
Your mistakes should be corrected right now.
The door may be locked inside.
Your homework can be handed in tomorrow.
6. 现在进行时的被动语态构成：am / is / are + being + 及物动词的过去分词
Uncle Wang is mending my bike now.→
My bike is being repaired by Tom now.
They are planting trees over there. →
Trees are being planted over there by them.
7. 不定式的被动语态：to + be + 及物动词的过去分词
There are two books to be read. →
There are twenty more trees to be planted.
三、 使用被动语态应注意的几个问题：
1. 不及物动词无被动语态。
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。
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 him. →He can’t be laugh 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.