It is difficult to tell one from the other ,when standing side by side后面when standing side by side 是什么用法类似还有you should take a map when travelling,when travelling是什么用法,什么时候用
问题描述：
It is difficult to tell one from the other ,when standing side by side
后面when standing side by side 是什么用法
类似还有you should take a map when travelling,when travelling是什么用法,什么时候用
答
when travelling时间状语
答
when引导的时间状语从句啦