如果主句和狀語從句的主語一致,狀語從句可以略去主語改寫為 “連詞 + 分詞短語”.

本句的 be careful 是祈使句,主語是省略的 you,從句 when you cross the street 中的主語也是 you,所以可以改寫如下：

Be careful when crossing the street.

另如：

He is very strict with them, whilehe loves his students. 雖然他愛他的學生, 可是他對他們很嚴格.

　　-- 可改寫為 He is very strict with them, while loving hisstudents.

I'll do it if I die in the attempt.即使會喪命我也要試一下.

　　-- 可改寫為  I'll do it if I dying in the attempt.