doing短语在句中作状语时，句子的主语与doing是主动关系，与谓语动作几乎同时发生。

    一、表示时间。可还原为含主动语态的时间状语从句。例：

    1.Crossing theroad,the old man wasknocked over by a car.

    When he wascrossing the road, theold man was knockedover by a car.

    2.Having fin-ished the homework,he went for a walk.

    After he hadfinished the homework,he went for awalk.

    说明：如果动作先于谓语动作发生，则用完成式：having done。

    二、表示条件 ......