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，listento，lookat，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。
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