一、肯定句改否定句
1、be动词后加not,诸如:is not(isn't), are not(aren't), was not(wasn't), were not(weren't), am not(无缩写)。
He is a student. 他是一个学生。--- He is not a student. 他不是一个学生。
They were watching TV then. 那时他们正在看电视。--- They weren't watching TV then. 那时他们没在看电视。
2、情态动词等后加not,诸如:can not(can't), will not(won't),should not(shouldn't),must not(mustn't),need(needn't)等。
She can take care of herself. 她能照顾好她自己。--- She can not(can't) take care of herself. 她不能照顾好她自己。