so that 后面跟目的状语从句。in order to和 so as to后面跟动词原形，in order to可以放在句首但 so as to不行。
You must speak louder so that /in order that you can be heard by all.
The doctors gave her a general checkup so as to/ in order to find out the cause of her illness.
The boy is so young that he can't go to school.
属于目的状语从句:从句部分是用以补充说明主句中谓语动词发生的目的的状语从句。
举例：
Say it louder(so)that everyone can hear you.
大声说，以便大家都能听到你的声音。
So that在目的状语从句中可表示"为了"的意思;在结果状语从句中表示结果。
目的状语从句:从句部分是用以补充说明主句中谓语动词发生的目的的状语从句。
表示目的状语的从句可以由that, so that, in order that, lest, for fear that, in case等词引导;
目的状语从句的谓语常含有may, might, can, could, should, will, would等情态动词。
一些用法：
1) so … that…/ such ….that
该句式中重要考查的是so和such的区别。应注意以下可能出现的结构:
so nice a boy that…
such a nice boy that…
so beautiful/diligent/carefully … that…
such (fine weather/great improvement) that…
2) so that
She worked hard so that everything would be ready by 5 o'clock.
她努力工作，以便在五点前是一切都就绪。
She looked down so that she should not see his eyes.
她垂下目光，使她看不到他的眼睛。
Everyone lend a hand, so that the sowing was done in time.
该句式中主要考查so that 与其它短语的区别，如:
由于so that也可以引导目的状语从句，同学们应特别注意从句中时态的运用。
3) so much so that
该结构用于形容词或副词之后，表示"到如此程度以致于"。如:
He was very weak, so much so that he could not walk.
I long to visit Beijing, so much so that I dream about it every night.
4) 注意结果状语从句与too….to..enough to…, so… as to结构的不同。
5)in order(that)"以便" to the end that "为……起见，以便"