是带连词when 的分词短语作状语。
如果与主语是主动关系 就用现在分词 如果是被动关系就用过去分词。
如：when getting off the bus ,you muat be careful.
when asked ,he said nothing.
扩展资料
例句：
When was the bridge constructed?
那座桥是何时修建的？
When did you get engaged ?
你们什么时候订的婚？
Until when can you stay?
你可以待到什么时候？
