Grammar and punctuation often bring a rolling of the eyes and a sigh. Boring! might be the word most people attribute to these most important elements of our writing world. Well ladies and gents, they may be boring but they're also mighty important.
I'd like to hone in on one tiny part of the whole punctuation picture. That is the comma used after an introductory clause or phrase in a complete sentence. I do a lot of critiques in my online writing group and that tiny comma is one place I find myself making a correction very often. I am amazed at how many writers omit the comma in this instance.
Look at this sentence:
Incorrect: When I left the beach the sun had dipped down to meet the waves.
Correct: When I left the beach, the sun had dipped down to meet the waves.
In the correct sentence, note that the part of the sentence after the comma could stand alone. That first phrase is not needed but it does add something to the sentence, doesn't it? Try reading both sentences aloud. Which one is easier? The comma in the second one gives you a place for a slight pause, a nanosecond of a breath, which makes it much easier to read aloud. In the first one, the whole thing runs together.
Even when only using a couple of words as an intro, the comma is needed. Look at the two sentences below:
Incorrect: That evening I slipped into bed hours later than usual.
Correct: That evening, I slipped into bed hours later than usual.
Once again, the second part of the sentence--the part beyond the comma--can stand alone. The two intro words tell us more. They let us know when.
For a little more detailed look at using a comma after an intro phrase or clause, look at this page or google the topic.
That tiny little comma makes a whale of a difference when it is placed in a sentence or when it is omitted.