I had to get up early so that I could catch the first bus.改为同义句,为什么,请说出解析,

I had to get up early in order to catch the first bus.
in order to do sth 表目的

i had to get up early ,so i could cathch the first bus
将一个从句改为两个分句，以so 替换 so that

I had to get up early so that I could catch the first bus.
= I had to get up early in order that I could catch the first bus.
= I had to get up early so as to catch the first bus.
= I had to get up early in order to catch the first bus.
= I had to get up early to catch the first bus.
so that可以等同于in order that,引导目的状语从句,且从句一般含有情态动词；
而so as to do sth等同于in order to do sth或to do sth,都视同于不定式引导目的状语.