Our editors often find the need to edit between two meanings. They try to make the right assumption after examining the context. What is left when they are done is clearer and more precise. It is often amazing how punctuation can make all the difference.
Introductory phrase with comma. An adverbial or participial phrase at the beginning of a sentence is usually followed by a comma, especially if a slight pause is intended. Often the comma eliminates dangerous or embarrassing ambiguities.
- After opening the present, Matilda turned pale.
- Exhausted by the morning's gift-giving frenzy, she lay down for a nap.
- Having eaten the Christmas goose, Fido devoured the mince pie.
- Before eating, the guests, children, and servants were treated to a speech by Father.
- To Anthony, Blake remained an enigma.
Introductory phrase without comma. A comma is not used after an introductory adverbial or participial phrase that immediately precedes the verb it modifies.
- Running along behind the sleigh was Scritzer himself.
- Out of the sleigh stepped the man we were looking for.
"Oh" and "ah." A comma follows an exclamatory oh or ah only if a slight pause is intended. No comma follows vocative oh or (mainly poetic and largely archaic) O.
- "Oh, what a beautiful mornin' ..."
- Ah, here's your fruitcake at last.
- Oh no! Ah yes! Oh yeah?
- Oh mighty king!
- "O wild West Wind ..."
Direct address. A comma follows names or words used in direct address and informal correspondence.
- Mrs. Claus, you've got a lovely daughter.
- Friends, I am here to inspect your Christmas cookies.
- Ma'am, the FedEx shipment arrived.
- Dear Santa,
"Yes," "no," and the like. A comma usually follows yes, no, well, and the like, at the beginning of a sentence if a slight pause is intended.
- Yes, I admit that the neighbor's lights are gaudier.
- No, we will not get up at 5 a.m. Christmas morning.
- Well then, we shall have a vote.
- No no no!
Source: The Chicago Manual of Style, 15th edition.