The Rule:

A comma goes after a phrase or clause that comes before the independent clause.  

Terms to Know:

Dependent Clause: Contains both a subject and a verb but is not a complete thought

Independent Clause: Contains both a subject and a verb and is a complete thought

Phrase: Contains a subject or a verb, but not both

Types of Introductions and Examples:

Single-word sentence modifier:  one word that modifies the meaning of an entire clause

Surprisingly, the roof was still intact.

• Surprisingly modifies the independent clause: the roof was still intact

Dependent Clause: Begins with a subordinating conjunction (words like after, although, as, when, while, until, unless, before, because, if, since) and doesn’t express a complete thought.

If they want to do well on the test, students must spend time studying.

• “If they want to do well on the test” contains a subject (they) and verb (want), but it doesn’t express a complete thought. It begins with a subordinating conjunction (if). Comma goes at the end of the dependent clause and before the main clause: students must spend time studying.

Infinitive Phrase:  Begins with the infinitive form of the verb (to + base form of verb)

To stay in shape for competition, athletes must exercise every day.

• Begins with to + verb (stay) and doesn’t contain a subject. Comma goes at the end of the entire infinitive phrase and before the main clause: athletes must exercise every day.
• Be careful of sentences where the infinitive phrase is actually acting as the subject of the sentence: To start a new business without doing market research and long-term planning in advance[x] would be foolish. (No comma here as it isn’t an introductory phrase; it is the subject of the sentence.)

Participial phrase: Begins with a past or present participle (–ed or –ing form of a verb that functions as an adjective)

Throwing caution to the wind, she told him she loved him.

• Begins with a present participle (throwing). The comma goes at the end of the entire participial phrase and before the main clause: she told him she loved him.
• Be careful not to confuse a participle with a gerund (an –ing verb functioning as a noun): Singing and dancing at the same time is hard to do. (No comma since singing and dancing aren’t functioning as adjectives; they are functioning as nouns, the subject of the sentence.)

Prepositional phrase: Begins with a preposition

Over the Christmas holiday, I cleaned out my attic.

• Begins with a preposition (over). Commas goes at the end of the prepositional phrase and before the main clause: I cleaned out my attic.
• You don’t have to have a comma after a short (2–3-word) prepositional phrase, though you can choose to.

Absolute phrase: Has a noun and modifiers (often includes a participle, but not always)

Their tummies satisfied, they crawled into bed.

• Their tummies (noun), satisfied (participle). Comma goes at the end of the entire absolute phrase and before the main clause: they crawled into bed.

Practice:

Put in the commas. Bonus if you can say what type of introduction it is.

1. Although we had reviewed the film twice before we never noticed these details about the shooting.
2. To get everything done I had to start earlier than usual.
3. Fortunately I was there to catch him.
4. By evening we had become impatient.
5. On the other hand I do agree it could use some work.
6. Dancing to the music I forgot all of my troubles.
7. When he says he will do it he means it.
8. Dancing around the room to my favorite song always makes me feel better.
9. Teeth clenched, eyes closed she braved the tall slide.
10. To be the best was all he could think about.

Answers:

1. Although we had reviewed the film twice before, we never noticed these details about the shooting. (Type of intro: dependent clause)
2. To get everything done, I had to start earlier than usual. (Type of intro: infinitive phrase)
3. Fortunately, I was there to catch him.(Type of intro: single-word modifier)
4. By evening we had become impatient. (could put a comma after “by evening” but don’t have to since it is a short prepositional phrase)
5. On the other hand, I do agree it could use some work. (Type of intro: prepositional phrase)
6. Dancing to the music, I forgot all of my troubles. (Type of intro: participial phrase)
7. When he says he will do it, he means it. (Type of intro: dependent clause)
8. Dancing around the room to my favorite song always makes me feel better. (no comma as “dancing around the room to my favorite song” is the subject of the sentence)
9. Teeth clenched, eyes closed, she braved the tall slide. (Type of intro: absolute phrase)
10. To be the best was all he could think about. (no commas as “to be the best” is the subject of the sentence)

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