如何把句子改成被动语态（英语）1.They often clean their classroom after scho

their classroom are often cleaned by them after school.
keys were often used by us for locking doors.
主动句是指该句主语是谓语所表示的动作行为的发出者的句子.被动句则是指该句主语是谓语所表示的动作行为的承受者的句子.
主动句和被动句互相转换的要点是主语和宾语的转换：主动句变被动句时,将主动句的宾语变为被动句的主语,主动句原来的主语与“被”构成介宾短语作被动句的状语；被动句变主动句则恰好相反.
（四）主动句变被动句
1.主动句变被动句第一步：要看时态,时态必须保持一致.第二步：把主动句的宾语改为被动句的主语,谓语动词由主动形式改为被动形式,主动句的主语成为介词by的宾语,放在句尾.
For example:
People keep sheep for producing wool.
Sheep _______ ______ for producing wool _______ people.
答案解析：在主动句中people 是主语,是动词keep动作的执行者,sheep就是keep的承受者.在变被动语态时,把主动句中的宾语sheep变成被动句中的主语,主动句中的主语变被动句的宾语,其前加介词by.谓语动词变过去分词.故正确答案为：are,kept,by.
2．主动句改为被动句,要注意谓语动词由主动形式变为被动形式时的各种变化,具体有以下几种变化：
一般现在时的被动语态：am/is/are + 过去分词
一般过去时的被动语态：was/were + 过去分词
现在完成时的被动语态：have/has +been + 过去分词
情态动词的被动语态 ：can/may/must/．．．+be + 过去分词
一般将来时的被动语态：will/shall +be + 过去分词
现在进行时的被动语态：am/is/are +being + 过去分词
过去进行时的被动语态：was/were +being+ 过去分词
过去完成时的被动语态：had + been+ 过去分词
接上面的：

their classroom are often cleaned by them after school.
keys were often used by us for locking doors.

passive voice is the opposition to the active voice by exchanging the position of the Subject and Object

their classroom is often cleaned by them after school.
keys were used to lock doors by us.找出动作的接受者，把它变成主语，再把动词改成被动式，方法和汉语的被动句差不多