Here's a breakdown:
When to use a comma:
* When "so that" introduces a dependent clause: This clause cannot stand alone as a complete sentence and provides additional information about the main clause.
* Example: *She studied hard, so that she could get good grades.*
* When "so that" is used to clarify or emphasize something:
* Example: *We have to leave now, so that we can get to the airport on time.*
When to omit a comma:
* When "so that" is used in a simple, short phrase and doesn't create a complex sentence structure:
* Example: *She worked hard so that she could buy a new car.*
* When "so that" introduces a direct object or an adverbial phrase:
* Example: *He explained his plan so that everyone understood.*
* When "so that" is used as part of a series:
* Example: *He asked her to meet him at the coffee shop so that they could talk and so that he could apologize.*
In doubt?
If you're unsure, it's always best to use a comma. It's better to be safe than sorry and potentially create a grammatically incorrect sentence.