英语主动语态怎么改成被动语态

主语和宾语换个位置 动词前面加be动词 动词变成被动形式（通常后面加ed）
如果主语是动作的承受者,或者是说动作不是由主语而是由其他人完成的,则用被动语态.主动语态→被动语态：
1、找到该句宾语（必须是在谓语动词后的）,将它提到句子开头.
2、用被动语态结构be+Vt过去分词构成句子.
3、把原句的主语用by构成的介词短语带出,也可不加.
eg：I finished my homework yesterday.
→My homework was finished (by me) yesterday.
特殊形式：
1、部分Vi加上适当介词可当作Vt构成被动语态.
eg：Parents look after their children carefully.
→Children were looked after (by parents) carefully.
2、联系动词没有被动语态.eg：The water feels warm.
3、在主动语态中一些省略to的动词不定式改为被动语态时需加上to.
eg：Teachers often make him stand outside.
→He is often made to stand outside (by teachers).
4、have、let等动词无被动语态.
必要时,have可改为held,let sb to do sth可改为be allowed to do sth (被允许做某事）.

简单来说假设A是主语，B是宾语
那么主动为A do B（do 为动词）
被动则为B be done by A
有时by A 可以省略

主语与宾语的位置互换 谓语改成被动形式：
e.g:
Jim beat mary.
Mary was beaten by Jim.