Introductory clauses are dependent clauses that provide background information or "set the stage" for the main part of the sentence, the independent clause. For example:
If they want to win, athletes must exercise every day.
(introductory dependent clause, main clause)
Because he kept barking insistently, we threw the ball for Smokey.
(introductory dependent clause, main clause)
Introductory clauses start with adverbs like after, although, as, because, before, if, since, though, until, when, etc.
Introductory phrases also set the stage for the main action of the sentence, but they are not complete clauses. Phrases don't have both a subject and a verb that are separate from the subject and verb in the main clause of the sentence. Common introductory phrases include prepositional phrases, appositive phrases, participial phrases, infinitive phrases, and absolute phrases.
To stay in shape for competition, athletes must exercise every day.
(introductory infinitive phrase, main clause)
Barking insistently, Smokey got us to throw his ball for him.
(introductory participial phrase, main clause)
A popular and well respected mayor, Bailey was the clear favorite in the
campaign for governor. (introductory appositive phrase, main clause)
The wind blowing violently, the townspeople began to seek shelter. (introductory absolute phrase, main clause)
After the adjustment for inflation, real wages have decreased while corporate profits have grown. (introductory prepositional phrases, main clause)
Introductory words like however, still, furthermore, and meanwhile create continuity from one sentence to the next.
The coaches reviewed the game strategy. Meanwhile, the athletes trained on the Nautilus equipment.
Most of the evidence seemed convincing. Still, the credibility of some witnesses was in question.
When to Use a Comma
Introductory elements often require a comma, but not always. Use a comma in the following cases:
After an introductory clause. (Does the introductory element have a subject and verb of its own?)
After a long introductory prepositional phrase or more than one introductory prepositional phrase. (Are there more than five words before the main clause?)
After introductory verbal phrases, some appositive phrases, or absolute phrases.
If there is a distinct pause. (When you read the sentence aloud, do you find your voice pausing a moment after the introductory element?) to avoid confusion. (Might a reader have to read the sentence more than once to make sense of it?)
When not to Use a Comma
Some introductory elements don't require a comma, and sometimes the subject of a sentence looks like an introductory element but isn't. Do not use a comma in the following cases:
After a brief prepositional phrase. (Is it a single phrase of less than five words?)
After a restrictive (essential) appositive phrase. (See our document on appositives.)
To separate the subject from the predicate. (See below.)
Each of the following sentences may look like it requires a comma after the opening segment (marked with an x), but the opening segment is really the subject. It's sometimes easy to confuse gerund- or infinitive-phrase subjects like the following with nonessential introductory phrases, so be careful.
Preparing and submitting his report to the committee for evaluation and possible publication[x] was one of the most difficult tasks Bill had ever attempted.
To start a new business without doing market research and long-term planning in advance[x] would be foolish.
Extracting the most profit for the least expenditure on labor and materials[x] is the primary goal of a capitalist.
Commas After Introductory Phrases
Use a comma to separate a group of prepositional phrases of more than four words when the phrases come at the beginning of a sentence.
Do not use a comma between separate phrases unless they are in a series.
A comma may also set off a single prepositional phrase at the beginning to make the sentence clear. A comma is recommended after any introductory prepositional phrase of more than four words.
Correct: Under the kitchen table the dog cowered.
(Single short, clear phrase. No comma needed.)
Correct: Under the spreading chestnut tree, the village smithy stands.
(Comma optional, but helpful due to length of phrase)
Correct: Under the pile of clothes, we found his wallet.
(Two prepositional phrases, not in a series)
Incorrect: On the sand, of the beach, by the inlet, we relaxed in the sun.
(Do not separate the phrases since they are not in a series.)
Correct: On the sand of the beach by the inlet, we relaxed in the sun.
Correct: Over hill, over dale, we hit the dusty trail.
(The two phrases are in series here. We could say "Over hill and over dale.")
Introductory Participial and Infinitive Phrases
Use a comma to separate introductory participial phrases and infinitive phrases used as modifiers.
Correct: Looking for help, the man fell on his knees to beg.
Correct: To raise enough money in time, Mary had to issue stock in her business.
(The infinitive phrase is used as a modifier)
Incorrect: To ski, is exhilarating.
(The infinitive is used as a noun, not a modifier.)
Correct: To ski is exhilarating.