动词的语态
语态有两种：主动语态 和 被动语态。
主语是动作的发出者为主动语态；
主语是动作的接受者为被动语态。
1）若宾语补足语是不带to 的不定式，变为被动语态时，该不定式前要加 "to"。
此类动词为感官动词: feel, hear, help, listen to, look at, make, observe, see, notice, watch
The teacher made me go out of the classroom.
--> I was made to go out of the classroom (by the teacher).
We saw him play football on the playground.
--> 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。
Coal can be used to produce electricity for agriculture and industry.
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。
They let the strange go.--- > The strange was let go.
2） 若let 后宾补较长时，let 通常不用被动语态，而用allow或permit 代替。
The nurse let me go to see my classmate in the hospital.
---- > I was allowed / permitted to see my classmate in the hospital.
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。
This is a photo of the power station that has been set up in my hometown.
My sister will be taken care of by Grandma.
Such a thing has never been heard of before..
3. 表示"据说"或"相信" 的词组
believe, consider, declare, expect, feel , report, say, see, suppose, think, understand
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
It is said that she will leave for Wuhan on Tuesday.
4. 不用被动语态的情况
1) 不及物动词或动词短语无被动语态：
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.
After the fire, very little remained of my house.
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：
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
This key just fits the lock.
Your story agrees with what had already been heard.
3) 系动词无被动语态：
appear, be become, fall, feel, get, grow, keep, look, remain, seem, smell, sound, stay, taste, turn
It sounds good.
4) 带同源宾语的及物动词，反身代词，相互代词，不能用于被动语态：
die, death, dream, live, life
She dreamed a bad dream last night.
5) 当宾语是不定式时，很少用于被动语态。
(对) She likes to swim.
(错) To swim is liked by her.
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, drive…
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
I was to blame for the accident.
Much work remains.
3) 在need, require, want, worth (形容词), deserve后的动名词必须用主动形式。
The door needs repairing.= The door needs to be repaired.
This room needs cleaning. 这房间应该打扫一下。
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)，have sth. done ( 要某人做某事)。
6. 被动形式表示主动意义
be determined, be pleased, be graduated (from), be finished, be prepared (for), be occupied (in), get marries
Eg: 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. 都可。
He married a rich girl.
He got married to a rich girl.
7. need/want/require/worth
注意：当 need, want, require, worth(形容词)后面接doing也可以表示被动。
Your hair wants cutting. 你的头发该理了。
The floor requires washing. 地板需要冲洗。
The book is worth reading. 这本书值得一读。
典型例题
The library needs___, but it'll 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 clean 则也为正确答案。
典：done，"不可能已经"。must not do 不可以（用于一般现在时)。
来源：https://www.cnblogs.com/Hiberniane/archive/2010/05/21/2488681.html