三、 主动语态变被动语态的方法

 1. 把主动语态的宾语变为被动语态的主语。
 2. 把谓语变成被动结构(be＋过去分词) (根据被动语态句子里的主语的人称和数，以及原来主动语态句子中动词的时态来决定be的形式)。
 3. 把主动语态中的主语放在介词by之后作宾语，将主格改为宾格。
例如：All the people laughed at him. (变为被动语态)→ He was laughed at by all the people.
They make the bikes in the factory.(变为被动语态)→ The bikes are made by them in the factory.
He cut down a tree.(变为被动语态)→ A tree was cut down by him.

四、 含有情态动词的被动语态

含有情态动词的主动句变成被动句时，由“情态动词＋be＋过去分词”构成，原来带to的情态动词(如have to/ought to) 变成被动语态后“to”仍要保留。
We can repair this watch in two days.→
This watch can be repaired in two days.
They should do it at once.→
It should be done at once.