I've learned that commas must be put before and, but, so, ect. However, I came across this sentence in a reading test.
Exercise causes blood vessels to open up so blood flows more easily.
Why is there no comma before so? Must I put it there?
Punctuation – Is a Comma Necessary Before ‘So’?
commaspunctuation
Best Answer
The comma in question separates coordinate clauses.
It's not the conjunctions themselves that require this comma. It's the compound sentence structure that requires both a comma and a coordinating conjunction. The words "and", "but" and "or" function only as coordinating conjunctions, so compound sentences joined by them will always* include the comma.
The words "so", "for" and "yet" sometimes function as coordinating conjunctions, but they also have other functions. "So" is an interesting word because it sometimes functions as a subordinating conjunction.
Your example sentence uses "so" as a subordinating conjunction.
With the comma, we have a compound sentence:
Here, the two clauses are independent. Each clause could stand as a separate sentence.
Without the comma, we have a complex sentence:
Here, "blood flows more easily" is subordinate. The phrase "so blood flows more easily" modifies the phrase "to open up".
There is practically no difference in meaning between the complex and compound versions of that sentence. The difference might be easier to see given a different example:
In the first, we know the reason that he's having a party. In the second, we have no idea.
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* This "always" isn't quite true. Some style guides suggest that short, well-balanced clauses don't need this comma. It's confusing but it's common.**
** The clauses "it's confusing" and "it's common" are short and well balanced.