If a dependent clause (one that cannot stand alone as a complete sentence) precedes an independent clause, a comma is needed.
If an adverbial phrase begins a sentence, it needs to be followed by a comma. (An adverbial phrase modifies a verb, adjective, adverb, or entire sentence.)
- After Rudy stepped up to the plate, he promptly tripped over it.
- As far as I'm concerned, you should cease that infernal whistling at once.
If the adverbial phrase is short, a comma is not necessary. Use the comma if it helps to clarify the intended meaning of the sentence.
- From the behind the curtain, we caught a glimpse of the leading actor preening his mustache before he made a grand entrance.
- Because of Junior's mumps, we had to cancel the Annual Family Fish Fry and Funkfest.
The introductory element in these examples gives more information about the main thought. The information of these examples could be given at the beginning or at the end of the sentence without changing its basic meaning. Certain types (when information could come at beginning or end) of introductory phrases introduce the main thought without modifying it, so a comma is needed. When the information given modifies the verb that immediately follows it, you do not use a comma. Look at the following rule and examples.
- At sunset the sewage line broke and spewed its darkness across the land.
- Before lunch the ducks arrived in all their iridescent glory.
If an introductory adverbial phrase immediately precedes the verb it modifies, then a comma is not used.
If an infinitive phrase modifies a noun or a verb, it should be set off by a comma. (Word groups consisting of to + a present tense verb + object are infinitive phrases.)
- Out of the bushes appeared a well-dressed man with his head underneath his arm.
- In the hallway danced a bevy of folk maidens with flowers and snakes in their hair.
If an introductory participial phrase functions as an adjective or an adverb, it should be set off by a comma. (A word group that consists of verb ending with -ing, -ed, -en + object is a participial phrase.)
- To get the rest of it off your chest, you would have to remove your shirt.
Watch for introductory participles that are not part of a longer introductory phrase. These follow the introductory phrase rule and therefore need a comma.
- Entering the room, she found herself ignored.
- Given a choice, I would much rather sleep than study.
- Watched by police, the shoppers grew increasingly paranoid.
Be aware of -ing verbs masquerading as nouns (gerunds). There is no comma after the gerund when it is the subject of the sentence.
- Running, she sped on down the lane.