例句：Tom is in the classroom.
否定句：Tom isn't in the classroom.
含be动词的肯定句做否定句时，是在be动词的具体形式后加not。又如：
We are students.---- We aren't students.
一般疑问句：Is Tom in the classroom?
肯定回答：Yes，he is.
否定回答：No, he isn't.
含be动词的陈述句改一般疑问句时，是将be动词的具体形式提前到句首。又如：
They're very happy. ---- Are they very happy?
i am a boy
i am not a boy
are you a boy
yes,i am
no,i am not
否定句:NO,they aren't
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