1.当我们的焦点集中在受事者或所发生的事情上,而施事者不重要,或显而易见,我们就用被动语态:
She was taken to the hospital at once. 她立刻被送往医院。
The People's Republic of China was founded on October 1， 1949. 中华人民共和国成立于1949年10月1日。
在被动结构中,如果要强调施事者,就用by短语:
The letter was written by Tom yesterday. 信是汤姆昨天写的。
2.不及物动词没有受事者,所以没有被动语态。break out（爆发）,exist（存在）,rise （升起）,fall（落下）,happen（发生）,occur （发生）,take place（发生）等都不用于被动语态。
3.有些及物动词由于其词汇意义,很少用于被动语态。它们是:elude?穴使想起）,flee（逃离）,get（获取）,have（有）,let（让),race（与……赛跑）,resemble（相似）,suit（合适）,survive（幸存于）等。例如我们不说:*He was survived in the shipwreck. 而说:He survived the shipwreck. （在这次沉船事故中他幸免于死。）
4.以不定式作宾语的动词,大多数只有主动语态而没有被动语态:
He always forget to wash behind his ears.他总是忘记洗耳后跟的地方。(*To wash behind his ears is always forgotten. )
少数动词如aGREe， decide， desire， arrange 等可有被动语态;但不把不定式直接置于句首,而用先行词it:
We desired to work and achieve success.
It was desired to work and achieve.
我们希望工作并取得成功。
He decided to appoint a new manager.
It was decided to appoint a new manager.
他决定任命一位新经理。
arrange， decide， expect， hope， manage， offer， propose， threaten 等动词后面的不定式，如果是及物的,不定式的宾语在双重被动句中作主语是很常见的:
The Trojans managed to bring the wooden house into the city.
The wooden house was managed to be brought into the city.
特洛伊人设法把木马搬进城里。
He expected to sell his house before May.
His house was expected to be sold before May.
他指望五月前把房子卖出去。
5.用于“s + v. + that clause” 句型的动词,有些也有被动语态,如acknowledge， assume， believe， claim， estimate， feel, find， know， presume， report， say， think， understand等。它们的被动语态,有两种形式:
1）用形式主语it 代替that从句,处在主语的位置上:
People believed that she was innocent.
It was believed that she was innocent.
人们认为她是无辜的。
Many years ago， it was widely believed that intelligence， special talents， insanity and criminality were all more or less inborn. 多年前,很多人相信:智力、特殊才能、精神病、犯罪,或多或少是天生的。
It is recognized that a university must do more than supply the facts of medicine， law， engineering. 人们公认:大学要作的事,不仅仅是提供医学、法律学、工程学方面的知识。
It has long been assumed that agGREssion is caused by male hor-mones. 长期以来有这么一种说法:冲劲是男性荷尔蒙造成的。
2）用that 从句里的主语作被动句的主语:
Some said that he was the richest man in the world.
He was said to be the richest man in the world.
有人说他是世界上最富有的人。
注意这种转换要正确运用时态:
People believe that she has written three books.
She is believed to have written three books.
人们认为她写了三本书。
We estimate that a fire in Hollywood in 1961 cost them U.S. ＄5，000，000.
A fire in Hollywood in 1961 is estimated to have cost them U.S.＄5，000，000.
1961年好莱坞发生了一场大火,据估计他们损失了五百万美元。
Some said that several persons had been killed in the accident.
Several persons were said to have been killed in the accident.
据说在事故中有几个人丧生。
3）注意suppose 用于主动与被动结构时的不同意义:
I suppose you know where the key is. 我想你知道钥匙在哪里。
“be supposed to do sth.” 含“责任、义务”等情态意义:
You are supposed to know where the key is. 你应该知道钥匙在哪里。
