he will have to stay in hospital. that's what he must do .这是正确的句子,我想问,如果是
问题描述：
he will have to stay in hospital. that's what he must do .这是正确的句子,我想问,如果是
that's what he must be doing ,就怎么错了
答
因为前面描述的行为,也就是stay in hospital还没有发生,所以不可以是正在进行时态.