第1个回答 2017-03-22
如果句子的谓语本身含有be动词，直接在be后加not即可。例如：
She is goot at science.改为否定：She is not(isn't) good at science.
She is reading a book.改为否定：She isn't reading a book.
如果句子的谓语含有助动词have（用于完成时。注意是助动词，不是实意动词），直接在have后面加not即可。例如：
She has read this book.改为否定：She has not(hasn't) read this book.
She had finished the housework when I came back home.改为否定：She hadn't finished the housework when i came back home.
当句子含有情态动词时（can, could,must, should, shall, will, would, may,might等），直接在情态动词后加not。例如：
You can do it tomorrow.改为否定：You can't do it tomorrow.
但需注意，情态动词must表示必须做某事的时候，其否定不用mustn't（mustn't表示一定不能，而不是“不必”，而应该用needn't）。如：
Must i finish the task today?——No, you needn't.我必须今天完成任务吗？——不，不必今天。
如果陈述句是除了上述的情况以外的，也就是说句子的谓语不含有be、have、情态动词，而是其他实意动词时，改成否定就需要在动词前补充助动词do（需要根据时态和人称变换），然后再加not。
例如：
She likes reading books.改为否定：She does not like reading books.(此时还需注意，likes改为否定句时，应用动词原形)
She went shopping yesterday. 改为否定：She did (过去时)not go shopping yesterday.(went同样要改为原形)
They like reading books.改为否定：They do not (don't) like reading books.