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。
