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.
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。
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.
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。
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..
12.3 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.
12.4 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.
