11.情态动词+行为动词完成进行式

    情态动词+行为动词完成进行式（即情态动词+ have been + v-ing 形式），表示推测或评论过去某动作是否正在进行或一直在进行。例如：

    1）They should have been meeting to discuss the problem.

    2）He may / might have been buying stamps in the post office when you saw him.