英语肯定句变否定句：be+not，情态动词+not，do或does+not。
1、be动词的否定式
be动词根据不同的人称和时态有不同的形式，在一般现在时中是am、is、are可用作联系动词，构成否定式时，一律在其后面加否定词not。
He is reading.——He is not reading.
They are from China.——They are not from China.
2、情态动词的否定式
情态动词的否定式是在其后面直接加not，如：
I can swim.——I can't swim.
He can dance.——He can't dance.
You should go to school at seven.——You shouldn't go to school at seven.
3、实义动词的否定句
实义动词变否定句时，要借助助动词do、does，在一般现在时中用do或者does其句子结构为：主语+don’t或doesn’t+动词原形+其它，例如：
I like pop music.——I don't like pop music.
He likes running.——He doesn't like running.
She does her homework at home.——She doesn't do her homework at home.
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