当我们推测已经发生的事情的时候,我们使用must have done.如：
Look!Tom is playing computer games.He must have finished his homework.Otherwise his mother won't let him do so.看!汤姆玩游戏呢.他作业肯定做完了.否则他妈是不允许他这么做的.
The ground is wet.It must have rained last night.地面是湿的.昨晚一定下雨了.