9.3 連詞+分詞(短語)
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  有時(shí)為使分詞短語與主句關(guān)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(gè),如:
  While waiting there, he saw two pretty girls come out of the building.
  waiting 和saw 的主語相同。