3) 作状语：
现在分词短语可以表示一个同时发生的次要的或伴随的动作：
Following Tom, we started to climb the mountain.
Opening the drawer, he took out a box.
Taking a key out of his pocket, he opened the door.
现在分词短语还可以表示原因, 相当于一个原因状语从句：
Not knowing her address, we couldn’t get in touch with her.
