由when , while , if , unless , though等連接詞所引導的副詞子句與主要子句的主詞相同時,副詞子句 動詞改為現在分詞,可以省略 S(主詞) 及 be-V(be動詞) 

例:

Though the man has a lot of money, David is not happy.

=Though having a lot of money, David is not happy.

例:

Brad broke his left leg while he was skiing in Canada.

=Brad broke his leg while he skiing in Canada.

例:

When completing two years later, the building will offer both office and residential space.

=When it completes two years later, the building will offer both office and residential space.