Question

he+is+singing+now改为否定句？

Answer 1

He is singing now.
英语中陈述句变否定句要注意以下三个方面：
1、陈述句中的谓语部分有be动词、助动词或情态动词，直接在这些词后加not即可。
如:
He is a student.
He is not a student.
He has studied English for 10 years.
He has not studied English for 10 years.
He can speak English.
He can not speak English.
2、陈述句的谓语部分只有实义动词。要分情形而定。
1）陈述句为一般现在时，主语不是第三人称单数。在谓语动词前加don't。
I study English every day.
I don't study English every day.
2）陈述句为一般现在时，主语是第三人称单数。在谓语动词前加doesn't，原陈述句的谓语动词改为原形形式。
He studies English every day.
He doesn't study English every day.
3）陈述句为一般过去式。在谓语动词前加didn't，原陈述句中的谓语动词改为原形形式。
He studied English yesterday.
He didn't study English yesterday.
3、肯定句中的some在否定句中应改为any
如：There are some students in the classroom. There are not any students in the classroom.
题目
He is singing now.中有助动词is（构成现在进行时），因此本句属于第一种情形（本题不涉及第三种情况），在will 后面加not即可。
He is not singing now.

Answer 2

He isn't singing now.
在be动词is后面直接否定就可以了。

Answer 3

he must be sleeping now改为否定句？ 这个句子的意思是他现在必须睡觉。那么按照汉语的否定意思，也就是他没必要马上睡觉。即He need not be sleeping now. (此时的need是情态动词，所以用动词原形。)切记，must的否定一般用 need not. 而不是mustn't.因为mustn't是禁止的意思。所以一定要注意句子要表达的意思

Answer 4

这个句子的意思是他现在必须睡觉。那么按照汉语的否定意思，也就是他没必要马上睡觉。即He need not be sleeping now. (此时的need是情态动词，所以用动词原形。)切记，must的否定一般用 need not. 而不是mustn't.因为mustn't是禁止的意思。所以一定要注意句子要表达的意思

Answer 5

He isn't singing now