連詞+分詞(短語)
　　有時為使分詞短語與主句關系更清楚，可在分詞前加連詞。 連詞有： when，while，if though，after, before, as.　但分詞的主語和主句的主語必須為同一個，如：
　　While waiting there, he saw two pretty girls come out of the building.
　　waiting 和saw 的主語相同。

情態動詞+have+過去分詞
1) may(might) have + done sth, can (could) have + done sth　表示過去，推測過去時間里可能發生的事情。
　　Philip may (might) have been hurt seriously in the car accident.
　　Philip can (could) have been hurt seriously in the car accident.
2)　must have +done sth，對過去時間里可能發生的事情的推測，語氣較強，具有"肯定"，"諒必"的意思。
　---Linda has gone to work, but her bicycle is still here.
　---She must have gone by bus.
3) ought to have done sth, should have done sth
　　本應該做某事，而事實上并沒有做。否定句表示"不該做某事而做了"。
　　You ought to (should) have been more careful in this experiment.
　　He ought not to have thrown the old clothes away.（事實上已扔了。）
　　ought to 在語氣上比should 要強。
4) needn't have done sth　 本沒必要做某事
　　I dressed very warmly for the trip, but I　needn't have done so. The weather was hot.
5) would like to have done sth　 本打算做某事　
　　I would like to have read the article, but I was very busy then.