二、由肯定句变为否定句
由肯定句变为否定句的规则是在句中的助动词之后添加否定词not。如果句中的谓语动词的情况和上述1、2、3点相同，则直接在be动词、情态动词和have、has或had之后添加否定词not 。除了be动词的am词形外，否定词not可以和助动词缩写为“助动词 + n't”。而如果句中的谓语动词是行为动词的一般现在时或一般过去时，则要先添加助动词do、does或did，再在其后添加否定词not，分别可以缩写为don't、doesn't和didn't 。
注：在行为动词一般现在时和一般过去时的否定句中，行为动词也一律为动词原形。
eg.(1) He is from England .
→He isn't from England .
(2) They can look for things on the Internet in Computer lessons .
→They can't look for things on the Internet in Computer lessons .
(3) I have seen the interesting film already .
→I haven't seen the interesting film yet .
(4) His brother often plays foogball after school .
→His brother doesn't often play football after school .
(5) My grandma came for dinner yesterday evening .
→My grandma didn't come for dinner yesterday evening .
下面几种句型在变为否定句时情况特殊一些，做题时须引起注意：
(1) I think + 宾语从句。
该句型在变为否定句时，否定词not应放在主句上，即为：I don't think + 宾语从句。
eg. I think it will rain tomorrow .
→ I don't think it will rain tomorrow .
(2) 祈使句
祈使句的特征是没有主语，以动词原形开头。在变为否定句时，应在动词原形之前添加助动词don't .
eg. (1) Water the flowers .
→ Don't water the flowers .
(2) Please clean the blackboard .
→ Please don't clean the blackboard .
