用现在分词的完成时态,表示分词的动作prepare发生在主句动作(decided)之前,相等于说：
Because the doctor had not prepared, he decided to postpone the operation.
如果直接用现在分词,分词的动作和主句的动作是同时发生的,例如：
Taking the umbrella, she rushed out of the door.那为什么不用made 呢如果用过去分词，则表示这个分词的逻辑主语（即主句的主语）和这个过去分词之间是被动的关系：Punished harshly by his mom, the girl cried hard.