pleace they must have finished their work by now.为什么用have finished?
they finally succeeded in carrying out the experiment.
they must have finished their work by now.为什么用have finished?
i have been looking forward to hearing from you.为什么用have been?
it has not been decided yet when the meeting will start at the hall.为什么用been,不要be?
i won't visit that restaurant today.won't 在什么时候用?