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So and so that differ in this way:
So that (often shortened to so) answers the question Why:
[1] Max ate dinner early so that she could go for a run.
Let's test it
Question:
Why did Max eat dinner early?Answer:
So that she could go for a run.
[2] Max ate dinner early so (that) she could go for a run.
Let's test it
Question:
Why did Max eat early?
Answer:
So (that) she could go for a run.
Elsewhere use , so. (Note the comma (,) there. It's important. , so means, and so, as a consequence, consequently, therefore):
[3] Max ate dinner early, so she could go for a run.
Let's test it
Max ate dinner early; therefore she could go for a run.
Does that help?
Try here, http://www.chompchomp.com/handouts/commatip07.pdf