On doing 作状语

On doing 作状语
（1）As soon as she saw it,the woman said she did not like it.
=>Seeing it,the woman said she did not like it.
（2）As soon as he opens the gate from the outside,he comes into the garden and waits until the gate shuts.
=>On opening the gate from the outside,he comes into the garden and waits until the gate shuts.
(3)The moment you leave this tent,you will get surprise.
=>On leaving this tent,you will get surprise.
为什么（1）用doing,（2）（3）用On+doing呢?
(4)The vicar has been asked to have the tree cut down.He has refused so far.
=>Having been asked to have the tree cut down,the vicar has refused so far.
(5)The tree was planed near the Church 50 years ago.It has gained an evilreputation only recent years.
=>Planed near the Church 50 years ago.It has gained an evilreputation only recent years.
为什么（5）可以直接用被动语态作状语,而（4）用现在分词的过去完成时的被动语态做状语?
原来如此，可是为什么要可省略as soon as呢？
到底什么时候可以省略句首的关系代词，什么时候不能省略呢？
1) When it is seen from the top of the hill,the city looks very small indeed.
=>Seen from the of the hill,the city looks very samll indeed.(省略)
2)When the problem is seen from this perspective,it is not complicated at all.
=>When seen from this perspective,it is not complicated at all.
3)When he was asked what he was doing,he smiled guiltily and then put the parcel on the desk.He explained that his diet was so stict that he had to reward himself occasionally.
=>When asked what he was doing,he smiled guiltily and then put the parcel on the desk.He explained that his diet was so stict that he had to reward himself occasionally.（没省略）

切记,on doing这个固定结构是表示”一.就.”,跟”as soon as”完全同一个意思!（1）也可以说成On seeing it,the woman said she did not like it.
除了（2）和（3）之外的句子考察的都是”分词表伴随”这个用法．注意,分词并不是简单的doing．其中,”伴随”就是伴随状态．是用现在分词还是过去分词,这取决于分词这个动作与主句主语的关系,是主谓关系就用现在分词,动宾关系就用过去分词．此外,分词的时态也要跟句子时态一致,这就是为什么（4）那里用了过去分词的完成时态,因为原句用了完成时态．