如果主句和状语从句的主语一致,状语从句可以略去主语改写为 “连词 + 分词短语”.
本句的 be careful 是祈使句,主语是省略的 you,从句 when you cross the street 中的主语也是 you,所以可以改写如下：
Be careful when crossing the street.
另如：
He is very strict with them, whilehe loves his students. 虽然他爱他的学生, 可是他对他们很严格. -- 可改写为 He is very strict with them, while loving hisstudents.
I'll do it if I die in the attempt.即使会丧命我也要试一下. -- 可改写为 I'll do it if I dying in the attempt.