1.一般时：表示的动作与谓语动作同时发生。
When we arrived，we found him sleeping.我们到达时发现他在睡觉。
Living in the suburbs，we have few visitors.因为住在郊外，我们没什么访客。
2.完成时：表示的动作发生在谓语动作之前。
Having cleaned the desks，we began reading.擦完桌子后，我们便开始看书。
Not having finished his homework，the boy didn’t go to sleep.
因为没有完成作业，这男孩没有睡觉。