关于when引导的从句

关于when引导的从句
When it realized how different the world seemed when it rolled so quickly,it stopped.句子中为什么要用两个when,分别是什么作用呢.为什么用seemed这个动词,又该怎么翻译,作用是什么.

大的框架是：When it realized...,it stopped.（when引导时间状语从句）
how different the world seemed when it rolled so quickly是realized的宾语,也就是how引导的宾语从句
在这个结构里又嵌有一个由when引导的时间状语从句when it rolled so quickly
句子意思的发展顺序是：
it rolled so quickly
→it realized how different the world seemed
(=it realized that the world seemed quite different)
→it stopped
seem是系动词,意味“看起来...；似乎是...”
当它意识到“自己转得很快时,世界会变的很不寻常”时,它停下来了.