The question of when to use a comma with so vs. so that vs. so…that isn’t normally tested on any standardized test I’m familiar with, but I’ve noticed a lot of confusion about it in various people’s writing recently, and so I wanted to address it here.
Essentially, the issue is that while all three constructions involve the word so, they’re actually three different types of conjunctions, and that in turn affects how they are punctuated.
So… (pun intended), here goes:
1) So by itself – synonym for therefore
So is a coordinating (FANBOYS) conjunction that serves to connect two independent clauses (complete sentences). Like the other FANBOYS conjunctions (for, and, nor, but, or & yet), so must follow a comma when it is used this way.
Incorrect: The skin is located at the interface between our body and the outside world so its cells can respond to many different kinds of stimuli.
Correct: The skin is located at the interface between our body and the outside world, so its cells can respond to many different kinds of stimuli.
2) So that – synonym for in order to
So that is a subordinating conjunction that separates an independent clause from a dependent clause.
When the independent clause comes first—as it virtually always does in this case, for stylistic reasons—a comma should not be used because it creates an unnecessary and unnatural break between the parts of the sentence.
Incorrect: People who spend long hours in the sun are encouraged to wear long-sleeved clothing, so that they limit their skin’s exposure to harmful rays.
Correct: People who spend long hours in the sun are encouraged to wear long-sleeved clothing so that they limit their skin’s exposure to harmful rays.
3) So…that – used to indicate extremity/cause and effect
So…that is a correlative conjunction (word pair), used with an adjective. Again, using a comma with this construction creates an unnecessary and awkward break.
Incorrect: The sun at the beach was so strong, that we were forced to leave only two hours after we had arrived.
Correct: The sun at the beach was so strong that we were forced to leave only two hours after we had arrived.