想当初的谁是谁非，如今早已物是人非。

Like the original of which is right and which is wrong now already have changed.

想当初，16岁那年，我试着溜进她的睡袋，她一脚把我踢了出来！

I'd tried to get in to her sleeping bag once when I was 16 but she kicked me out!

想当初他们漂洋过海来到这陌生的土地上，开始自己的学习生活是何等艰辛呀！

You can't imagine how hard it was when they traveled across the sea to this land.