1.主系表结构的句子
I am a teacher. ----- I am not a teacher.
He is in Beijing.----- He is not in Beijing.
We are classmates.----- We are not classmates.
It was cold yesterday.----- It was not cold yesterday.
这些句子中都有系词，am，is，are，was，were，只要在系词后面+not ，就变成否定句了。
2.主谓宾的句子
He runs fast.----- He doesn't run fast. 主语是三单人称，do也要用三单形式does
They learn from me.------ They don't learn from me.
We went to school by bike .----- We didn't go to school by bike.
谓语动词是过去式，do也要变过去式did
这些句子中都有谓语动词，只要在主语后面+do的适当形式+not 就行了。
3.句中有助动词或
情态动词的
I will get up earlier tomorrow.----- I will not get up earlier tomorrow.
She can dance well.----- She can't dance well.
He has gone abroad.------ He has not gone abroad.
只需要在助动词或情态动词后面直接+not ，就可以变否定句了。
OK？希望能帮到你啊！