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 ...