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。
