So = The intensive so means ‘very or extremely’ (The teachers’ attitude is so casual these days) occurs chiefly in informal speech. We use so in place of so that even in formal writing; hence, the matter is stylistic preference.
So that = To the amount or degree expressed or understood; to such an extent; used to introduce a clause giving the reason for or purpose of an action; hence, most authorities and grammarians insist that so must be followed by that in formal writing.
Both so and so that are used to introduce clauses that show a result or consequence:
He failed to appear in the court, so the judge pronounced him guilty in his absence. (with the result or consequence that)
I took a cab so I could catch up with you. (with the purpose that)
He folded his umbrella so he could pass through the narrow lane. (in order to make something happen)
In informal situations, we usually omit that after so:
We stayed up late so (that) we could watch the match till end.
We built a fire so (that) we could keep ourselves warm through the night.
She has made some Yorkshire Pudding so (that) we can have something on the way.
The school has planned a variety of activities, so there’ll be something for every age group.
Pamela moved her car so Jack can get his into the garage.There is no milk, so you’ll have to drink black tea. (something is true as a result of the situation just stated)
In formal writing, so that is used more often than so in clauses of purpose. Anyways, both so and so that are standard.
Greater flamingo return every year around February, so that March is a good time to see them.
She is now waiting for the rain to stop so that she will leave for market.
We were so relieved to learn that the last date for submitting the admission forms had been extended.
She was so overwhelmed by all the flowers and letters of support that she cried for a long time.
Why don’t you start out early so that you don’t get caught in the traffic?
She’s brushing up on her English so that she can enroll in English-speaking universities.
Some concrete measures should be taken so that this kind of calamity never happens again.
We booked our seats early so that we could get entry into the stadium.
When we talk about the future, we can use the present simple or will/’ll after so that.
He will do the class project on pollution today so that you submit it to the school tomorrow. (or … so that you will submit it …)
In order that = with the purpose that; so that We also use in order that to talk about purpose. It is often used with modals (can, could, will, would etc.).
Notice that in order that is more formal: The court delivered him the notice by hand in order that he could receive it without fail. (or … so that he could receive)