Commas after sentence introductions
 
When words which "set the scene" for the main part of the sentence appear at the front of the sentence, it is usual to offset them with a comma. For example:
  • At 4 o'clock, I'll jump in the river.
  • As I know the ground, I'll go first.
  • Under the water, you can see the swans' legs going hell for leather.
When the introduction is just one word (e.g., Yesterday, Here, Now), it is common practice to omit the comma.
 

Commas after Introductions

It is common for a sentence to start with an introduction. An introduction can be anything from just one word to a long clause. In general, an introduction is used to state a time, a place, a condition, a frequency or a fact before the main part of the sentence. (Introductions vary hugely.)

Examples:

In the centre of London, the number of people who fell victim to pickpockets 
rose by 30 per cent in a month. (sets a place)

After twelve years of therapy, my psychiatrist said something that brought tears to my eyes. He said, "No hablo ingles." (sets a time)

In ancient Rome, it was considered a sign of leadership to be born with a crooked nose. (sets a time and place)

As soon as the cake is golden-brown, take it out of the oven. (sets a time)

From the moment I picked your book up until I laid it down, I was convulsed with laughter. Some day, I intend reading it. (both set a time) (Groucho Marx quote)

Yesterday, the manager visited the stables. (sets a time)

When I was young, I used to think that money was the most important thing in life. Now that I am old, I know it is. (both set a time) (Oscar Wilde quote)

On Tuesday 4th July a band played carols in the park for 8 hours.
("On Tuesday 4th July" sets a time. It is an introduction and should be 
followed by a comma.)

Having spoken to John, I can confirm that the meeting is definitely off.
(states a fact)

As you are well aware, the latest figures do not look promising.  
(states a fact)

After the secretary had read the minutes of the meeting, the chairman 
asked for the financial report.
(sets a time)

A band played in the park for 8 hours on Tuesday 4th July.
("On Tuesday 4th July" sets a time, but it is not an introduction - it's at the back end of the sentence.)

On Tuesday 4th July, a band played in the park for 8 hours .
(sets a time)

If you are going through hell, keep going. (sets a condition) (Winston Churchill quote)

 
ONLY USE A COMMA FOR AN INTRODUCTION 

Words that "set the scene" do not always start the sentence; they could appear at the back or in the middle. A comma is only expected when these words appear at the front of the sentence. This is very handy to know. Look the examples below:

At 4 o'clock, the new manager, David Bain, will visit.
(correct, but too many commas)
The new manager, David Bain, will visit at 4 o'clock.
(correct — much tidier)

Here is an example in the middle of a sentence:

I'm very proud of my gold pocket watch. My grandfather, on his deathbed, sold me this watch. (Woody Allen quote)

Commas have been used in the example above, but they were not necessary because the phrase that sets the scene appears in the middle not at the beginning.

A LOT OF LENIENCY ON THIS RULING

Regardless of where the scene-setting words appear (start, middle or end), there is some leniency on whether to use a comma or not.  The primary purpose of a comma (or commas if it appears in the middle) is to aid your reader.  Above all else, making your text easy to read should determine whether you use a comma or not.

DON'T USE A COMMA AFTER A ONE-WORD "INTRODUCTION"

Most of the scene-setting words are classified as adverbs. When they are made up of more than one word, they're called adverbial phrases or clauses. When they're just one word, it is a common practice to drop the comma. For example:

  • The day before yesterday, I caught another 10lb bass.
  • (A comma is expected after an introductory adverbial phrase.)
  • Yesterday I caught another 10lb bass.
  • (A comma after "Yesterday" (because it's just one word) would look a bit unwieldy, so it's okay to omit it.)
 

See also:

Commas after a transitional phrase
Commas after interjections (yes, no, indeed)
Commas before conjunctions (and, or, but)
Commas for parenthesis
Commas in lists
Commas with a long subject
Commas with numbers
Commas with quotation (speech) marks
Commas with the vocative case
Commas with Dear, Hello, and Hi
List of easily confused words


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