The train which has just left is for xian .中left 后为什么跟 is.
问题描述：
The train which has just left is for xian .中left 后为什么跟 is.
which 做什么成分
答
is 是主句中的系动词
The train is for xian 这列车是开往西安的
which has just left 是定语从句
全句意为：刚开走的那列火车是开往西安的