请教英语达人，反义疑问句，请详解！

Question

1.they must have been very busy yesterdy ，（ ）they?
2.the boy wearing a red cap,come to the front,( )( )?
3.she can not have been to Africa in the past few years,()()?
4.what you need most at present is help,()()?
5.he must be helping the old man to water the folwers,()()?

Answer 1

they must have been very busy yesterdy ，（ wasn't）they?                      他们昨天一定非常忙，不是吗？
2.the boy wearing a red cap,has come to the front,(hasn't )(he )?

戴红色帽子的男孩已经来到的前面，不是吗？
3.she can not have been to Africa in the past few years,(have)(he)?

她不可能在过去几年里去过非洲，不是吗？
4.what you need most at present is help,(don't)(you)?

你目前最需要的是帮助，不是吗？
5.he must be helping the old man to water the folwers,(isn't)(he)?

他一定正在帮着老人浇花呢，不是吗？

追问

为什么？请详解？

追答

反义疑问句
是前否定，后肯定
而有虚拟语气结构的句子中，要根据时态来判断助动词或者be动词的形式。