时间状语从句与when引导的定语从句怎么区分?

关键是看前面有没有先行词,有就是定语从句,没有就是时间状语从句
如:1.By the time when you arrived in London,we had stayed there for two weeks.到你到达伦敦的时候,我们在那里已经待了两个星期.有先行词 the time 那么when you arrived in London就是定语从句
　1‘.when you arrived in London we had stayed there for two weeks.没有先行词 when you arrived in London就是时间状语从句