有时为使分词短语与主句关系更清楚，可在分词前加连词。 连词有： when，while，if though，after, before, as. 但分词的主语和主句的主语必须为同一个，如：
While waiting there, he saw two pretty girls come out of the building.
waiting 和saw 的主语相同。
While waiting there, he saw two pretty girls come out of the building.
waiting 和saw 的主语相同。
