题目内容：

独立主格结构在英语里的作用是什么?
当独立主格在英语中表示主语动作先后的时候,与过去完成时+过去式来表示动作的先后 有什么区别呢?
如 Hearing I had no money and would take none,people bought me food or shared whatever they happened to have with them.
能用had done+did 的形式互换?