Note: The following comes from our top-selling grammar book, Abrams' Guide to Grammar.
Commas Overview The comma is probably the most used, overused, and misused form of punctuation. For some reason, writers often suffer from comma angst. I have no magic formula for learning comma rules; they are many. Some may even be a judgment call. But even so, you must be able to go to a rule to support any change you make. Once you understand a rule for the comma—again that’s understand, not memorize— you will be able to make a generalization about the rule and apply it to your writing, editing, or proofreading.
Commas with Independent and Dependent Clauses
Two independent clauses can be connected with a comma and a coordinating conjunction (and, but, for, or, nor, yet, and sometimes so.) For example:
- She was a welcome guest in our home, and she always made us feel we were the perfect hosts.
Use a comma to set off an introductory dependent clause. A dependent clause contains both a subject and a verb, but it is incapable of standing alone as a sentence because of the word introducing the clause. Words that commonly introduce dependent clauses include when, that, as, if, unless, although, after, because, since, until, though.
- Because we cut the budget, we will not be able to hire any new staff members this year.
Do not use a comma to set off most adverbial dependent clauses at the end of a sentence unless the clause is parenthetical. Adverbial clauses answer one of the following questions: where? when? why? to what degree? in what manner?
- We will not be able to hire any new staff members this year because we cut the budget.
Note: Dependent clauses that begin with that, which, who, and whom are not adverbial clauses.
Commas After Introductory Constructions
Commas are used to set off most types of introductory constructions. Introductory constructions are words, phrases, or clauses used at the beginning of a sentence or at the beginning of an independent clause elsewhere in the sentence.
Set off an introductory dependent clause with a comma. We have already discussed setting off a dependent clause when it appears at the beginning of a sentence. A dependent clause introduced by a subordinating conjunction is usually not set off if it appears at the end of a sentence.
- Although punctuation may seem arbitrary, most rules are closely related to meaning.
Note that although makes the first clause dependent. The comma after arbitrary helps make the independent clause stand out.
Remember to place a comma after a dependent clause that introduces an independent clause in the middle of a sentence.
- I walked to the library to get Anita Shreve’s new book, but when I arrived, all copies had already been checked out.
Note that the preceding sentence begins with an independent clause. But joins the first independent clause with a second independent clause, which itself is made up of one dependent clause and one independent clause. A comma is placed after book because it introduces an independent clause and makes the independent clause stand out. Remember, more important information should be in the independent clause.
Use a comma to set off an introductory verbal phrase. A verbal phrase is a word group containing a verb form that is used as another part of speech and has no subject. There are three forms of verbals: the infinitive (to walk), which can be an adjective, adverb, or noun; the present participle (walking), which is always an adjective, and the past participle (walked), which is also always an adjective. As with dependent clauses, the use of the comma after the verbal phrase makes the independent clause stand out.
Here are examples of verbal phrases.
- Infinitive phrase: To learn to play the guitar properly, you must take lessons and practice every day.
- Present participial phrase: Offering tender loving care, the nurse was a favorite among the patients on the pediatric ward.
- Past participial phrase: Embarrassed by his ill-spoken words, he quickly changed the subject.
Use a comma to set off transitional words and phrases. Transitions are words or phrases at the beginning of a sentence that help connect the sentence to the preceding sentence.
Here are examples of transitional words and phrases.
- Finally, the seniors will participate in an all-night grad party.
- In the meantime, you may use the computer in the lab.
- As a result, we can all enjoy an extra day off.