牛津译林 模块六第二单元 M6U2 Grammar and usage语法
4.现在分词的两种时态：一般式doing, 通常指与主句的谓语动词所表示的动作 同时发生或无先后顺序; 完成式having done则强调分词所表示的动作先于谓语 动词所表示的动作。如： While walking in the street, we met some friends of ours. (同时发生) Having waited in the queue for half an hour, Tom suddenly realized that he had left his wallet at home. (“等待”在先)