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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。