53.No sooner had he sat down to lunch than there was a knock at the door.为什么用than.而不是when
问题描述：
53.No sooner had he sat down to lunch than there was a knock at the door.为什么用than.而不是when
答
因为sooner是副词soon的比较级，要用than引导出比较状语部分。
答
固定 一...就...
答
no sooner.than. 是固定搭配