回答:这要从两方面来谈这一问题:1.
分词短语作状语时,其前的
从属连词可以省略; 2.
状语从句中的主语与主句主语一致时,可以直接用分词短语作状语,也可以在分词短语之前加上连词,以示强调。例如:
(When ) Working in the city, he often went climbing the mountain.
( Though ) Knowing it, he didn't tell anyone.
( If) Having time, I will go on a visit to China.
( When ) Completed, the library will be open to the public.