He tied a complex knot so that others would find it hard to undo.
More Examples
Used in the Middle of Sentence
I need to collimate my telescope so that the images are clearer.
Here is an article that spells things out, so that it is easier to grasp.
We've been told by Whosers that have been to the tapings (such as Ashley!) that they tape more than an hour to get the 22 minutes or so that they actually use.
Meaning of so that for the defined word.
Grammatically, this idiom "so that" is a conjunction.