Take a look at this Beatles verse:
And then while I’m away,
I’ll write home every day,
And I’ll send all my loving to you.
It’s one sentence with three clauses. We call that first line a “dependent” clause, because it cannot exist alone as a sentence. It depends on other parts of the sentence. The second and third lines are “independent” clauses, because each can stand alone.
Commas separate and link things at the same time. Here commas divvy up the sentence, so the reader can see the parts. The comma after “away” in the first line tells the reader that the first (dependent) clause has ended, and predicts that another related clause will follow. The second and third (independent) clauses are separated and linked by a comma followed by the conjunction “and.” Commas act as road signs telling readers when one part of the journey ends, and what to expect in the next part.
A heavy punctuator (I’m one) might put a comma after “And then” to separate the introductory phrase for the reader. But remember that readers take commas as pauses, so a comma after “And then” might hurt the rhythm. Sometimes but rarely, we might put a comma between a complex subject and its verb, like this: “A comma after ‘And then,’ might hurt the rhythm.”
Our Beatles verse would make sense without the commas, although sentences without punctuation are harder and slower to read. Sometimes unpunctuated clauses bash into each other, like this: “The restaurant served ham and eggs and biscuits and gravy never appeared on the menu.” The reader doesn’t know where to split the clauses. Did the restaurant serve ham and eggs and biscuits, but never gravy? Or did it never serve eggs and biscuits and gravy? Or did it serve ham and eggs, but never biscuits and gravy? Commas don’t just enhance clarity; they create meaning.
Try this one: “Although the restaurant served ham and eggs biscuits and gravy never appeared on the menu.” The reader crashes into the combination “eggs biscuits” because the writer failed to separate the opening dependent clause (“Although the restaurant served ham and eggs”) from the independent.
We can join two independent clauses with just a comma in less formal writing: “The chef loved ham and eggs, his wife hated biscuits and gravy.” In more formal writing, we could use a semicolon: “The chef loved ham and eggs; his wife hated biscuits and gravy.” And we could break the sentence into two (“The chef loved ham and eggs. His wife hated biscuits and gravy.”) if we wanted to push the marrieds further apart.