@THX1138
Commas are simpler than you might think. I find the biggest problem is that there are a good many people with opinions on commas based entirely on their own whims. But much like spelling, there are standard conventions.
Commas are used in a number of ways, but for writers of fiction, there is only one major application: to separate ideas in order to make them clear.
Often a comma is used to separate phrases and clauses in a sentence. To fully grasp this use, one had to understand phrases and clauses, not commas.
a second common use is to separate items in a list. Some items might be just a single word while others might be two or more, so the comma lets you know where each ends. (Since we always put
and before the last one, there's controversy over whether a comma is also necessary. But I guess in Oxford they always do this.)
The third major use is appositives. These always need to be separated with commas so know where the appositive starts and ends.
So whenever you are writing and aren't sure if you need a comma ask yourself if you have one of these three cases and then add your punctuation accordingly.
There are a few exceptions to these conventions but that's usually where the comma is optional, so it's not wrong if you use one.
Regional differences do exist, but you can leave that to a copy editor.
Do not, however, fall for the often repeated "tip" that you should use a comma wherever you pause for a breath. It has no grammatical basis whatsoever and, depending on your lung capacity, may be wrong more often than right.