直接用过去分词开头作状语时不强调动作的时间,尽管这一动作有时也发生在谓语动词之前.只强调被动.如:Led by the Party,we have achieved great success.在党的领导下,我们取得了巨大的成功.
又如:Respected by the students,the teacher is very happy.受到学生们的尊重,老师很高兴.
当用Having +过去分词时,主要强调动作发生的时间是在谓语动词之前.
如:Having finished his homework,he went out to play with his friends.做完作业之后,他出去和朋友玩了.
至于找主语的问题,你只要知道主语是谓语动作的发出者,没有谓语动词的动作哪有什么主语,省掉是省掉的问题.这和非谓语动词不是一码事.