12. 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.