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， mak英语e， 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.