1．被动语态的构成
由于不及物动词不能带宾语，故无被动语态，只有及物动词或相当于及物动词的动词短语才有被动语态，其基本构成方式是“助动词be＋过去分词”。
注意：“be＋过去分词”结构不一定都是被动语态，有些动词（如 be，feel，look，seem等）后面的过去分词已转化为形容词，用作表语表示状态。如：
My bike is broken．（我的自行车坏了。）
The door is open．（门开了。）
2．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．）
3．被动语态改为主动语态的方法：
被动语态中介词by后的宾语改为主动语态中的主语（或按题意要求确定主语），按照这个主语的人称和数以及原来的时态把谓语动词形式由被动语态改为主动语态。注意在主动语态中有的动词要求不带to的动词不定式作宾语补足语，此时要把被动语态中的to去掉。被动语态的主语用来作主动语态的宾语。如：
History is made by the people．→ The people make history．