ing形式的动作与谓语动词的动作同时发生。
People waiting for the bus often shelter in my doorway. 等车的人经常挡我的路。
I like the girl sitting on the right. 我喜欢坐在右边的那个女孩。
ing形式的动作是习惯性动作。这种情况下名词中心词都为泛指,不能为特指。
Passengers traveling on this bus buy their tickets in books. 坐这班公车的旅客到售票处买票。
Anyone touching that wire will get an eclectic shock. 碰到那条电线的人会被电击。