in order that是表示目的,
如果句子是 Isent the letter early that morning___ she can get it that afternoon.明显表示目的,答案就是in order that .
但是句子中she got it 表示过去式,不是表示目的
so that 表示一种因果关系目的．比如：
she is so clever that she passed the exam esaily
she worked hard so that she can pass the exam .
上面的是过去式,这里就是因果关系．