怎么找非谓语的逻辑主语?

我们知道,动词的非谓语形式有三种：不定式、v-ing形式和v-ed形式.非谓语的逻辑主语,也就是在逻辑上存在的关系.它不像动词作谓语时,主语和动词存在主动或被动.
1.不定式.如：It is very hard for us to learn English well in a short time.此句中,to learn English well是主语,us 是动词不定式的逻辑上的主语.（是"我们"学习英语)
2.v-ing.如：His being late again made the teacher very angry.
又迟到了是他(his).
Arriving at a strange-looking house,he showed me into a large ,bright,clean room.来到的人是他（he)
一般来说,作状语的非谓语动词的逻辑主语就是句子的主语.
3.v-ed形式.如：Astonished at the amount of work in the new timetable,Lucy decided to leave her job immediately.
惊讶的人是Lucy,也就是说astonished 的逻辑主语是Lucy.