The most questions I get from my readers concern the comma. The comma intimidates writers (except author Steve Berry, the self-proclaimed Comma Kamikaze ☺). Unfortunately, correct comma usage is difficult to nail. The best I can offer comes from current acceptable standards found in updated reference materials.
In a sentence with two or more independent clauses, the comma must precede the coordinate conjunction (words such as and, but, for, so, yet, while, or, nor, and whereas).
She bought a new dress, and her sister bought a CD.
I do my reading and paper work in bed at night, while my brother stays up late watching Leno.
No comma is used before and, but, or, nor, and yet when the coordinate conjunction joins two words, two phrases, or two dependent clauses.
He has neither the willingness nor the ability to pay his bill.
Bravely, she stepped onto the stage but realized she couldn’t speak a word.
Why no comma in these two examples? The clauses share a single subject. He hasn’t the willingness to pay. He hasn’t the ability to pay. She stepped on stage. She realized she couldn’t speak. In either example, we see two actions tied to a single subject.
What happens when a second subject is introduced? We no longer have two dependent clauses:
You can make an appointment now, or we will call you later.
You is the subject of the first clause, and we is the subject of the second, bringing us back to the first scenario with two independent clauses.
Related to this is the couplet following a verb. In a simple sentence, the word and is sufficient.
They agreed to divide the cash and to put the stock in a trust fund.
Focusing on sentence structure is a good start to overcoming your comma-phobia. I’ve barely scratched the surface of the comma, so watch for future posts.