1、看句中有无be动词，如有，直接在be动词后+not。2、看句中有无情态动词，如有，直接在情态动词后+not。3、如上述二者都没有，就应用助动词+not。4、原句中动词假如发生变化就要恢复成原形。

肯定句变否定句变法规则

一、有be动词(am、is、are)和情态动词(can、could、may、must、shoud、will、would等)时，直接在be动词和情态动词后面加not，some变为any，and变为or，too变为either，其他的照抄。

例如:

①、I am a student. – I am not a student.

②、Amy is a good girl.–Amy is nota good girl.

③、These flowers are very beautiful.–These flowersare not very beautiful.

④、I can swim.–Ican't(can not)swim.

⑤、There are some books on the table.–There are not anybooks on the table.

⑥、Jack is a doctor , too.–Jack is not a doctor , either.

⑦、I was a student ten years ago.–I was nota student ten years ago.

注:⑴、 some 、and 用于肯定句中，any、or用于否定句和疑问句中。

⑵、was是is和are的过去式，were是are的过去式。

二、没有be动词(am、is、are)和情态动词(can、could、may、must、shoud、will、would等)时，在动词前面加don't、doesn't、didn't，动词恢复成原型，some变为any，and变为or，too变为either，其他的照抄。

例如:

①、I like math.–I don't likemath.

②、Jack likes math, too.–Jack doesn't like math , either.

③、They like bananas and apples.–They don't like bananas orapples.

④、Lily gets up at six every morning.–Lily doesn't getup at six every morning.

⑤、His father likes playing football.–His father doesn't likeplaying football.

⑥、I went fishing yesterday.–l didn't gofishing yesterday.

注:⑴、当句子是一般过去时态时，在动词前加didn't，动词恢复成原型。

⑵、当句子是一般现在时态时，主语是第三人称单数时，在动词前加doesn't，动词恢复成原型;主语不是第三人称单数时，在动词前加don't。