Commas and Introductory Elements

When a sentence begins with an adverbial clause, put a comma after it.

It is permissible, even commonplace, to omit a comma after most brief introductory elements — a prepositional phrase, an adverb, or a noun phrase:
When a prepositional phrase expands to more than three words, say, or becomes connected to yet another prepositional phrase, the use of a comma will depend on the writer's sense of the rhythm and flow of the sentence.
When an introductory adverbial element seems to modify the entire sentence and not just the verb or some single element in the rest of the sentence, put a comma after it.
Don't allow a brief introductory element to merge with something following it in a way that can confuse your reader. Try reading the following sentences without their commas:
When a sentence begins with an Absolute Phrase or an adverbial Infinitive Phrase, put a comma after it. (If the infinitive phrase is acting as a noun and is the subject of the sentence, be careful not to put a comma between the subject and its verb: "To believe in one's self is a good thing.")

Information in this section is based on material found in Understanding English Grammar by Martha Kolln. 4rth Edition. MacMillan Publishing Company: New York. 1994. p. 167-8. Examples our own.


Guide to Grammar and Writing