So that / So / So as to / In order /To
These phrases can be used to express purpose and can be removed to leave just "to".
If we use "in order to" and "so as to" it sounds a bit more formal and explicit than "to" by itself, but both are equally possible in both spoken and written English.
・In order to cut the tree down, I had to hack through the undergrowth first.
・To cut the tree down, I had to hack through the undergrowth first.
・He raised his voice so as to be heard.
・He raised his voice to be heard.
Before stative verbs like know, seem, appear, understand, have, etc, it is more usual to use "in order to" or "so as to":
・I talked to them both for half an hour so as to have a thorough understanding of the problem.
・I followed her around all day in order to know whether she had any intention of meeting him.
Before a negative infinitive, we normally use "so as" or "in order".
・In order not to oversleep, I set the alarm for seven o'clock.
(NOT: Not to oversleep)
・I am leaving now so as not to be late.
(NOT: not to be late )
These structures are used to talk about purpose.
"So that" is more common in informal speech and writing. Both these expressions are normally followed by "modal auxiliary verbs" such as "can (could) or will (would)".
In an informal style, "that" can be dropped after "so".
It wouldn't be recommended that you use it in a formal speech:
・He lowered his voice so that Doris couldn't hear.
・He lowered his voice in order that Doris couldn't hear.
・He lowered his voice so Doris couldn't hear.
・Why don't you start out early so that you don't have to hurry?
・Why don't you start out early in order that you don't have to hurry?
・Why don't you start out early so you don't have to hurry?
・I waited for an hour so that I could meet him.
・I waited for an hour in order that I could meet him.
・I waited for an hour so I could meet him.
Present tenses are sometimes used to refer to the future after so that/in order that.
・Send the letter today itself so that she gets/will get it before Friday.
・I am going to make an early start so that I don't/won't get stuck in the traffic.