We arrived early so that we could / should / might / would get a good seat.
so that 引导目的状语从句,既可以省略 so,也可以省略 that——前者(省略 so)属于正式用法(而且非常正式),后者(省略 that)属于非正式用法(而且相当不正式):
We arrived early so we could / should / might / would get a good seat.(相当不正式)
We arrived early that we could / should / might / would get a good seat.(非常正式)