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1．我们知道，主动语态变为被动语态时，通常将主动语态中的宾语变为被动语态的主语；主动语态的主语变为由介词by引导的宾语；谓语动词变为被动语态“be +过去分词”形式，例如：
Everyone likes the interesting film. (主动语态) → The interesting film is liked by everyone. (被动语态)
They used knives to cut things. (主动语态) → Knives are used to cut things. (被动语态)
但是，并非所有由“be +过去分词”构成的形式就是被动语态。有时过去分词已经变为形容词，表示一种状态。试比较：
My brother is very interested in English. 我哥哥对英语非常感兴趣。（状态）
My brother was interested by what I told him. 我告诉他的话让我哥哥非常感兴趣。（动态） 2．在被动语态中谓语动词一定是及物动词，也就是说必须跟宾语的动词。如：
We can make metals into different shapes. (主动语态) → Metals can be made into different shapes. (被动语态)
在主动语态中make是及物动词，其动作的承受者即宾语是metals。
但是，并非所有的及物动词都有被动语态。比如，表示静态的动词（have、belong to）、宾语是反身代词或相互代词等。例如：
He has a nice house. 他有一所漂亮的房子。
We must help each other. 我们应该互相帮助。 3．主动语态中的主语在变为被动语态时通常变为由介词by的引出的宾语。如：
They have already cut down the trees. → The trees have already been cut down by them. 那些树木已经被他们砍光。
如果谓语动词是know，在变为被动语态时，原来的主语变为由to引出的宾语。如：
May people know him very well. → He is well known to many people. 很多人都非常了解他。
注意：我们之所以用被动语态是因为我们不清楚动作的执行者、或没有必要指出动词的执行者等。因此很多被动语态没有由by引出的宾语。如：
People may lose their lives and homes or they may be injured badly in a fire. 在大火中人们可能失去性命和家园，要不就是严重烧伤。
It is used for protecting the fireman from smoke. 它用来保护消防人员以避免吸进烟雾。 4．在主动语态变为被动语态时，时态一般应保持一致。如：
We will build more factories in my hometown. → More factories will be built in my hometown. 我们家乡将要建起更多的工厂。
为此，我们必须记住以下某些时态的被动结构：
do（es）→ is/am/are done did → was/were done
have/has done → have/has been done will do → will be done
be doing → be being done 5．某些动词可以跟双宾语如give、pass、bring、write；buy、make等，在变为被动语态时如果直接宾语变为被动语态中的主语，则间接宾语通常由介词to、for引出。例如：
My sister gave me a nice watch for my birthday. → A nice watch was given to me by my sister for my birthday. 我过生日时收到了姐姐的一块手表。
They have bought us many books. → Many books have been bought for us by them. 很多书都是由他们给我们买的。
6．某些动词make、hear、see、watch等在主动语态中，其宾语补足语要求用不带to的不定式。但在变为被动语态时通常省略的不定式符号to必须恢复。如：
He made us clean the classroom after school. → We were made to clean the classroom after school. 我们被迫在放学后打扫教室。
I saw him come into the room. → He was seen to come into the room. 人们看见他进了屋子。
7．在主动语态中如果谓语动词是含有介词或副词的短语动词，变为被动语态时不能丢掉该介词或副词。如：
They must take good care of the children. → The children must be taken good care of. 孩子们必须小心照看。
8．英语中某些动词如sell、lock、draw、start等在否定句、进行时或用在某些形容词、副词前可以表示被动意义。如：
This kind of books sells well. 这种书畅销。
The door won’t lock. 门锁锁不上。
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