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.
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.
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 / 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