I was reading a book and stumbled on this sentence:
It was closed, but the salesman said he would wait, if we hurried.
I'm confused about the use of the comma preceding if we hurried.
Why not write
It was closed, but the salesman said he would wait if we hurried
Are these two sentences materially different? Do they convey different things? How do I decide whether or not to use a comma before a subordinate conjunction?