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.