一、简单句中含有be动词、情态动词和助动词时，变否定句时需在be动词、情态动词和助动词后加not。
1. （am, is ,are 是be动词）如：I am a teacher．→I am nota teacher．This is a pen.→ This is not a pen.或This isn’t a pen. (is not =isn’t) You are a boy. →You are not a boy. (are not =aren’t)2. （can是情态动词）He can help me．→He can not help me．或He can’t help me.(can not =can’t )3.当主语是第一人称、第二人称和第三人称复数时用助动词do,变否定时，在do后面加not 如：I like English .I do not like English.或I don’t like English.They go to school together. →They do not (don’t) go to school together.4.当主语是第三人称单数时用does,变否定时，在does后面加not，如：The little girl wants to go there.→ The little girl does not (doesn't ) want to go there．2．简单句中谓语动词为行为动词时，要根据时态选用do的相应形式，把肯定句变为否定句。
如：The little girl wants to go there．→The little girl doesn't want to go there．He did his work yesterday．→He didn't do his workyesterday．3．祈使句变否定句时，一般在句首加Don't，但Let us或Let's 开头的祈使句在Let us或Let's后加not也可。
如：Put them over there．→Don't put them over there．Let us go home．→Let us not go home．或Don’t let us go home.4．主从复合句中，主句是I think，I believe等结构，变否定句时，形式上否定主句，实际上否定从句。
如：I don't think chickens can swim ．我认为鸡不会游泳。
5．“had better＋动词原形”，变否定句时为“had better not ＋动词原形”。
如：You'd better go to school．→ You'd better not go to school．6．动词不定式变否定式时，常在不定式to前加not。
如：The teacher tells me to do it．→ The teacher tells me not to do it．7．使役动词和感官动词（make，let，hear，see，watch等）后接省略to的不定式，此种不定式变否定句时只需在省略to 的动词原形前加not。
如：I saw her work．→I saw her not work．8．特殊形式的否定句，有以下几种：（1）肯定句含有every-的合成词时，变否定句时改为no-。
如：Everything is ready．→Nothing is ready．（2）肯定句中含有always，usually和ever时，变否定句时改为never。
如：He is always late for school．→He is never late for school．（3）肯定句含有many，much时，变否定句时分别改为few，little。
如：Many students know him．→Few students know him．（4）肯定句中含有still时，变否定句时应改为no longer。
如：Tom is still in bed．→Tom is no longer in bed．（5）肯定句含有both，both．．．and，all时，变否定句时应改为never，neither．．．nor，none。
如：Both of us are students．→Neither of us is a student．（6）肯定句含有nearly，almost时，变否定句时应改为hardly。
如：Lily nearly knows him ．→Lily hardly knows him ．（7）肯定句含有“形容词＋enough”时，变为否定句时应改为“too ＋形容词的反义词”。
如：He is short enough to reachit．He is too tall to reach it．（1）谓语动词是be，have（有）或谓语含有助动词、情态动词时，直接在这些词后加上否定词not。
如：She can swim．She can't swim．（2）谓语动词是行为动词时，要在该动词前加上do ／does／did not，该词恢复原形。
如：He gets up early．He doesn't get up early．（3）既要否定谓语动词，又要变换句中的单词。
如：He can read and write．He can't read or write．（4）用含否定意义的词或词组取代肯定句中的某些词或词组。
如：I know both English and Chi- nese．I know neither English nor Chinese．类似的有：ever ／always never，somebody nobody，something nothing，everybody no one，almost hardly 2．祈使句的肯定式变否定式（1）一般在动词前加上don't。
如：Open the window ．Don't open the window．（2）含有“L et's．．．”的祈使句，一般用“Let'snot．．．”的形式，美国人偶尔也用“Don'tlet's．．．”这一形式。
加强语气时使用“Let'sdon't．．．”这一形式。
如：Let's go there．Let's not go there．3．复合句的肯定式变否定式一般将主句变为否定式。
如：I saw her when I left ．I didn't see her when I left．肯定句改为否定句的基本方法把肯定的陈述句改为否定句的基本方法首先要掌握否定句构成的基本规则。
①当陈述句的谓语动词由be构成时，改为否定句，把not 加在be后面。
例1．They are doctors．这个句子的谓语动词是由系动词构成的，改为否定句时把not加在are的后面，即：They are not doctors．例2．He is g oing to see his friend tomorrow．本句的谓语由is going to see构成，改为否定句时把否定词not 加在is后面，即：He is not going to see his friend tomorrow．②当陈述句的谓语由情态动词can（may，must等）＋其他动词构成，或由助动词will，have，had等十其他动词构成时，改为否定句，用情态动词或助动词十not＋动词的形式表示。
例3．They will go to see their parents this afternoon．这个句子谓语由助动词will加动词g o构成，改为否定句时，把not放在will后面，即：They will not（won't）go to see their parents this afternoon．例4．The train had already left when we got to the station．这个句子谓语由助动词had加leave的过去分词left构成，改为否定句时要把not加在had后面，同时already改为yet，即：The train had not left yet when we got to the station．例5．Linda can speak Chinese well．本句的谓语由can＋speak构成，把它改为否定句时，在can 后面加not，即：Linda can not speak Chinese well．例6．We have already learned English for two years．本句的谓语由have＋learned构成，把它改为否定句时，要在助动词have后面加not，把句中的already改为yet，即：We have not learned English for two years yet．③当陈述句的谓语动词是have，表示有什么，改为否定句的方法是：（A）由have＋not＋其他；（B）由have十no十其他；（C）由don't（doesn't，didn't）＋have十其他，如果句中有some，要将some改为any。
例7．You have some books．这个句子谓语动词have当“有”讲，在改为否定句时要把not放在have后，并把some改为any，即：You have not any books．这个句子也可以改写为在have后加no即：You have no books．这个句子还可以改写为：You don't have any books．④当陈述句的谓语由have十其他词构成，这时have不当“有”讲，也不是“助动词”，改为否定句时，由助动词do（does，did）＋not＋have构成否定结构。
例8．They often have breakfast at seven in the morning．本句的谓语动词是have，而have＋breakfast的意思是“吃早饭”，把它改为否定句，要由助动词do＋not＋have breakfast 构成否定形式，即：They don't have breakfast at seven in themorning．例9．He had a meeting yesterday afternoon．本句谓语动词had＋meeting的意思是“开会”，是一般过去时，改为否定句时由did not＋have a meeting构成否定形式，即：He didn't have a meeting yesterday afternoon．●以上两个例句说明改成否定句时，千万不要盲目把否定词加在have（has，had）后面。