语态有两种：主动语态和被动语态。主语是动作的发出者时为主动语态;主语是动作的接受者时为被动语态。
注意：1)感官动词主动语态的宾语补足语是不带to 的不定式，变为被动语态时，该不定式前要加to。例如：
The teacher made me go out of the classroom. 老师逼我走出教室。
-- I was made to go out of the classroom (by the teacher).
We saw him play football on the playground. 我们看见他在草场踢球。
-- He was seen to play football on the playground.
2)情态动词+ be +过去分词，构成被动语态。例如：
Coal can be used to produce electricity for agriculture and industry. 煤可以产生工农业需要的电。
12.1 let 的用法
1)当let后只有一个单音节动词，变被动语态时，可用不带to 的不定式。例如：
They let the strange go.他们放陌生人走了。
--- The strange was let go.
2)当let 后宾补较长时，let 通常不用被动语态，而用allow或permit 代替。例如：
The nurse let me go to see my classmate in the hospital. 那护士让我去探望住院的同学。
---- I was allowed / permitted to see my classmate in the hospital.
12.2 短语动词的被动语态
短语动词是一个整体，不可丢掉后面的介词或副词。例如：
My sister will be taken care of by Grandma. 我妹妹由奶奶照顾。
Such a thing has never been heard of before. 这样的事闻所未闻。
12.3 表示据说或相信 的词组，基本上由believe, consider, declare, expect, feel , report, say, see, suppose, think, understand等组成。例如：
It is said that 据说
It is reported that 据报道
It is believed that 大家相信
It is hoped that 大家希望
It is well known that 众所周知
It is thought that 大家认为
It is suggested that 据建议
It is taken granted that 被视为当然
It has been decided that 大家决定
It must be remember that 务必记住的是
12.4 不用被动语态的情况
1) 不及物动词或不及物动词短语，如appear, die disappear, end(vi. 结束), fail, happen, last, lie, remain, sit, spread,
stand, break out, come true, fall asleep, keep silence, lose heart, take place等没有无被动语态。
例如：After the fire, very little remained of my house. 大火过后，我家烧得所剩无几。
比较：rise, fall, happen是不及物动词;raise, seat是及物动词。
(错) The price has been risen.
(对) The price has risen.
(错) The accident was happened last week.
(对) The accident happened last week.
(错) The price has raised.
(对) The price has been raised.
(错) Please seat.
(对) Please be seated.
要想正确地使用被动语态，就须注意哪些动词是及物的，哪些是不及物的。特别是一词多义的动词往往有两种用法。解决这一问题唯有在学习过程中多留意积累。
2) 不能用于被动语态的及物动词或动词短语，如fit, have, hold, marry, own, wish, cost, notice, watch agree with, arrive at / in, shake hands with, succeed in, suffer from, happen to, take part in, walk into, belong to等。例如：
This key just fits the lock. 这把钥匙只配这把锁。
Your story agrees with what had already been heard. 你说的与我们听说的一致。
3) 系动词无被动语态，如appear, be become, fall, feel, get, grow, keep, look, remain, seem, smell, sound, stay, taste, turn等。例如：It sounds good. 听上去不错。
4) 带同源宾语的及物动词如die/death, dream/dream， live/life等，以及反身代词，相互代词，不能用于被动语态。例如：
She dreamed a bad dream last night. 她昨晚做了个恶梦。
5) 当宾语是不定式时，很少用于被动语态。例如：
(对) She likes to swim.
(错) To swim is liked by her.
12.5 主动形式表示被动意义
1)wash, clean, cook, iron, look, cut, sell, read, wear, feel, draw, write, sell等。例如：
The book sells well. 这本书销路好。
This knife cuts easily. 这刀子很好用。
2)blame, let(出租), remain, keep, rent, build等。例如：
I was to blame for the accident. 事故发生了，我该受指责。
Much work remains. 还有许多活要干。
3) 在need, require, want, worth (形容词), deserve后的动名词必须用主动形式。例如：
The door needs repairing.= The door needs to be repaired. 门该修了。
This book is worth reading. 这本书值得一读。
4)特殊结构：make sb. heard / understood (使别人能听见/理解自己)等。例如：
Explain it clearly and make yourself understood. 解释清楚些，让别人理解你的话。
12.6 被动形式表示主动意义，如 be determined, be pleased, be graduated (from), be prepared (for), be occupied (in),
get married等。例如：
He is graduated from a famous university. 他毕业于一所有名的大学。
注意：表示同某人结婚，用marry sb. 或get married to sb.均可。例如：
He married a rich girl. 他与一个富妞结婚了。
He got married to a rich girl.
12.7 need/want/require/worth
当 need, want, require, be worth后面接doing时，表示的是被动意义。例如：
Your hair wants cutting. 你的头发该理了。
The floor requires washing. 地板需要冲洗。
典型例题
The library needs___, but itll have to wait until Sunday.
A. cleaning B. be cleaned C. clean D. being cleaned
答案A. need (实意) +n /to do，need (情态)+ do，当为被动语态时，还可need + doing. 本题考最后一种用法，选A。如有to be cleaned 则也为正确答案。