There are two common ways to express that an action we do has a purpose: the particle to and the conjunction so that.
I bookmarked the webpage to view it later.(to view it later is the purpose)
I bookmarked the webpage so that I could easily find it later.(to find it later is the purpose)
to / in order to / so as to + infinitive
We say to + infinitive or, more formally, in order to + infinitive, so as to + infinitive when the same subject does the action and pursues the goal.
We need the key to unlock this door.We need the key if we want to open this door.
I'm skipping dinner to lose some weight.I'm skipping dinner because I want to lose weight.
In order to pass the exam, the students should revise all their notes.The students should revise them if they want to pass the exam.
The scientists conducted the experiments several times in order to avoid errors.They did it because they wanted to avoid errors.
If we need the negative, we say so as not to, less frequently in order not to.
I kept checking the meat so as not to overcook it.WRONG I kept checking the meat not to overcook it.
He set the alarm clock for 5 o'clock so as not to be late for his flight.WRONG He set the alarm clock for 5 o'clock not to be late for his flight.
so (that) + subject + verb
"So that", or its shorter informal version "so", connects two parts (clauses) of the sentence. That means we have a subject and a verb following "so that". It is common to put a modal verb after "so that" (for instance, won't/wouldn't or can/could), but not necessarily.
I'll come home early so that we can have dinner together.
Let's start the meeting at 10:30 so that we have enough time before lunch.
Put on oven gloves so that you don't get burnt.
Ben's father got a second job so that Ben could continue his studies.
Let's have a snack now so we don't have to stop on our way.