先看句子里有没有be动词（am,is,are）、情态动词(can,may,would),如果有,在这些词后面直接加not
如果没有,就在谓语动词前加don't,doesn't 或者didn't
例如：
He is a student.He isn't a student.
She can fly a kite.She can't fly a kite.
I go to school by bus.I don't go to school by bus.
He likes to play football.He doesn't like to play football.