请过路高人谈一下must be doing 与 must have been doing 的用法.这是在新概念第九课出现的.这里的 must 换成 shall 、could 、can 等意义上有什么变化吗?
问题描述：
请过路高人谈一下must be doing 与 must have been doing 的用法.
这是在新概念第九课出现的.
这里的 must 换成 shall 、could 、can 等意义上有什么变化吗?
答
he shall be doing...he could be doing..he can do...
must be doing- She must be doing her homework right now.
She must have been doing her chores for tomorrow..
答
must be doing也表示一般将来时和现在完成时的推测，must have been doing也表示一般过去式的推测，他们都是第二种表示推测的方法，而里的 must 换成 shall 、could 、can 等意义无任何变化。
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答
简单说一下吧..
must be doing:肯定某人正在做某事..He must be doing his homework.他一定在做他的作业
must have been doing:肯定某人一直在做的事..He must have been trying his best to study..他肯定一直都在尽力学习..
must表强烈的肯定,shall不那么强烈,could最弱..表推测的情况一般不用can ...