例如：
(1).Because he had seen the film before, he went out of the cinema.
删掉连词because和从句主语he，没有be动词，因此，把had seen变成having seen.
Having seen the film before, he went out of the cinema.
(2). When our school is seen from the mountain, our school is beautiful.
删掉连词when和从句主语our school,接着删掉be动词is，就成了
Seen from the mountain, our school is beautiful.