Ah, commas before “and”, you are my nemesis! But now I think I understand you better.

There’s the whole debate on Oxford/serial commas, which I will skip. Suffice it to say it’s the situation where you are using commas to delineate a list. Some say to put the comma before the “and”, some don’t, but either way, it’s a style thing and as long as you are consistent, it’s OK to do either.

But the problem I had when what to do when we’re not talking about a serial list.

From Get It Write I found this description:

The second situation occurs when “and” is being used to coordinate two independent clauses. An independent clause—also known as a main clause—is a group of words that has a subject and a verb and can stand alone as a sentence. In the following example, the independent clauses are in brackets:
  • [Miguel took piano lessons for sixteen years], and [today he is an accomplished performer].
The use of the comma would also apply when any of the seven coordinating conjunctions (and, but, or, nor, for, so, yet) join two independent clauses.
Notice in the next example that we do not use a comma before “and” because it does not join two independent clauses but merely joins two verbs:
  • Miguel took piano lessons for sixteen years and today is an accomplished performer.
Here we have only one independent clause—two verbs (“took” and “is”) but one subject (“Miguel”).