Understanding proper use of comma or no comma in a series
Thank you in advance for your help! I'm trying to understand what rules of grammar apply to a particular sentence. Here is the unruly sentence:
You may do A, but only if B or C happen to D.
My question relates to B. Is the following interpretation of the above correct? You may do A, but only on two conditions: (1) if D happens to B or (2) if D happens to C.
What are the proper rules of grammar in this instance? Does the meaning change if a comma is placed before the "or"?
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