英语肯定句∶
A duck can swim.
一只鸭子会游泳。
英语肯定句∶
A duck can't swim.
一只鸭子不会游泳。
将肯定句改为否定句的方法∶
在系动词be、助动词be，
have／has／had，will，shall，would，should和情态动词后加not（n't）。若句中没有这些词，则根据谓语动词的原形、第三人称动词形式或过去式而分别在其前面加do not（don't），does not（doesn't）或did not（didn't），并将第三人称动词或过去式改为原型。如：
（He）用含否定意义的词（如：never，nothing，nobody，none，neither，little，few等。
①always／evernever，andor。如：
肯定句∶He was always late for school last term．
否定句∶He was never late for school last term．
肯定句∶They have ever read the book．
否定句∶They have never read the book．
肯定句∶He can speak Chinese and English．
否定句∶He can't speak Chinese or English．
②too not…either，already not…yet。如：
肯定句∶He goes to school by bike，too．
否定句∶He doesn't go to school by bike， either．
肯定句∶He has already finished the work．
否定句∶He hasn't finished the work yet．
③allnone，bothneither，both…andneither…nor，此时，谓语动词仍用肯定形式，但当neither作主语，谓语动词用单数形式；neither…nor…连接两个主语时，谓语动词应和nor后面的主语保持一致。如：
肯定句∶He knows all of the workers here．
否定句∶He knows none of the workers here．
肯定句∶Both of the twins are from England．
否定句∶Neither of the twins is from England．
肯定句∶Both Emma and John have gone to Tokyo．
否定句：Neither Emma nor John has gone to Tokyo．
3.当think，believe，suppose等动词后面跟宾语从句时，通常用否定主句谓语动词来达到否定从句谓语动词的目的。如：
肯定句∶I think they can come tonight．
否定句∶I don't think they can come tonight．
4.当must表示推测，表示“一定、肯定”的含义时，否定句应用can't。例：
肯定句∶He must be in the room．否定句∶He can't be in the room．
肯定句∶He book in two months．
否定句∶He won't write the book in two months.
存疑请追问，满意请采纳，谢谢
（｡ò ∀ ó｡）