This is not a comprehensive list of comma rules. These are comma issues that I have found repeatedly in the many manuscripts I have edited.
Adjectives in a series—When you write something like The old proud warrior, you need to place a comma between the adjectives old and proud. You should separate two or more adjectives with a comma if each one modifies the noun alone. But if the first adjective modifies the idea expressed by the combination of the second adjective and the noun together, no comma should be used. For example: Barbara stood beside a tall blue spruce.
Adverbial clause—A comma is required before or after an adverbial clause, depending on whether it begins or ends a sentence, if it functions as a nonrestrictive clause. Example: When Tucker woke up a few hours later he didn’t know where he was. The adverbial clause is in boldface It’s a nonrestrictive clause in this sentence, so you should place a comma after the word later.
Comma before a person’s name—Example: My brother Jeff. Using a comma or not using one here changes the meaning. If you use a comma—My brother, Jeff—then you’re saying that you have only one brother, whose name is Jeff. Without the comma, the statement means that you have more than one brother, only one of whom is named Jeff.
Comma splice—If you link two sentences (two independent clauses, each with a subject and a verb) with a comma, you are committing an error known as the comma splice. Examples:
The current was swift, he could not swim to shore.
Martha was a lonely old woman, she moved to a new town.
In these constructions we have two separate independent clauses that express two separate thoughts. When you have two independent clauses, they must be written in one of the following ways:
• Separate them with a period so they’re treated as two complete sentences.
• Connect them with a semicolon if the two thoughts are closely related (I like you; you’re nice.)
• Connect them with a conjunction, which has a comma in front of it (The current was swift, and he could not swim to shore.)
Commas to avoid misreading— Sometimes you need to insert a comma for no grammatical reason but to avoid the misreading of the sentence. For example: In the place where the willow grew the river was broad and slow. This sentence could be easily misread. The reader may think you’re saying that the willow grew the river, unless you place a comma after the word grew. Some of these mistakes make you smile: When we had finished eating Jerry and I left the room.
Comma (s) with a conjunctive adverb—Incorrect: You must learn all the rules however if you want to be a good writer. A comma should be placed before and after the adverb however. A comma always follows a conjunctive adverb, and a comma precedes it if it is in the middle or at the end of a sentence. Conjunctive adverbs include accordingly, also, anyhow, as a result, besides, consequently, furthermore, henceforth, however, indeed, instead, likewise, meanwhile, moreover, nevertheless, otherwise, then, thus, too, and therefore.
Coordinate adjectives—You must place a comma between two or more coordinate adjectives if each one modifies the noun alone. Example: Uncle Charley looks like a warm, sensitive man. But if the first adjective modifies the idea expressed by the combination of the second adjective and the noun, no comma should be used. Example: Each band had a distinctive musical style.
Hot tip: You can check adjectives to see if they’re coordinate by applying two tests:
• Can you reverse the order of the adjectives without changing the meaning or creating nonsense? If yes, use a comma.
• Can you insert the word and between the adjectives? If yes, use a comma.
Introductory clauses and phrases—Some phrases and clauses that begin a sentence must be set off with a comma. Example (adverbial clause): When they got to the hospital the nurses took her to the emergency room. Here we could place a comma after the word hospital, at the end of the adverbial clause. Similarly, we need to insert a comma after the word waiting in this sentence: After hours of waiting the doctor came out to congratulate Robert.
Exceptions: In some cases the comma may be omitted after an introductory adverb clause and even a long introductory phrase if the omission does not lead to misreading. Example: As soon as I saw the deer I knew that I couldn’t shoot him. Omitting the comma after the word deer does not promote misunderstanding. In other cases, though, you should use a comma even if the introductory clause or phrase is short, mainly for clarity. Example: As we would expect, Freud’s self-evaluation would hardly be agreed upon by everyone.
Introductory words—Sometimes you have to place a comma after only one introductory word such as an adverb. Example: Fortunately, I didn’t catch the flu. You may also have to clarify the meaning, as you do with the word once: Once, I ate an apple vs. Once I ate an apple, I felt better.
Parenthetical elements—Any parenthetical elements (words, phrases) that are inserted into a sentence must be set off by commas. Example: Karen, not knowing what to do, ran to the kitchen. Nonrestrictive clauses, discussed elsewhere, are parenthetic, and so are clauses introduced by conjunctions indicating time or place (when and where). Commas are therefore needed. Deciding whether a word or a short phrase is or is not parenthetic can be difficult sometimes. As Professor Strunk says in his classic little book, The Elements of Style (which I highly recommend):
The writer may safely omit the commas if the interruption to the flow of the sentence is slight. Whether the interruption is slight or considerable, however, he must never omit one comma and leave the other. Examples:
Marjorie’s husband, Colonel Nelson paid us a visit yesterday.
My brother you will be pleased to hear, is now in perfect health.
Participial phrases—These phrases are modifying constructions that begin with verbs that end with -ing or -ed. They are most commonly found at either the beginning or the end of a sentence. Example: Sometimes Linda saw the old woman peeking between her curtains staring at her and Robert with her sad eyes. Here we need to insert a comma after the word window to set off the participial phrase staring at her and Robert with her sad eyes.
Serial comma—When you have a series of three or more elements with a single conjunction, place a comma after each element except the last one. This rule applies to single words, clauses, and phrases. In other words, the pattern you should use is A, B, and C, even though you’ll often see the pattern A, B and C, particularly in newspapers and news magazines. Either style may cause ambiguity, but the style that omits the comma is more likely to do so. Example: My favorite combinations are green and yellow, blue and purple and black and red.
Independent clauses/compound sentences
Every writer must learn how to identify the basic sentence unit—the independent clause—because such recognition is so fundamental to composition. Independent clauses have a subject and a verb, express a complete thought, and can stand alone. Example: Barbara shouted.
If two independent clauses are connected by a comma and a conjunction, they form a compound sentence. Example: Barbara shouted, and Steven looked around. Here we have two independent clauses connected with a comma and a conjunction—and. You must remember the comma when you write a compound sentence like this, and you must make sure that the second clause has both a subject and a verb.
Look at this sentence:
My parents tried their best to shield me from it but I was there.
Where are the independent clauses in this sentence (the subjects and verbs are underlined)? A comma should be placed after the word it, before the conjunction but, to separate the independent clauses:
Here’s another point to note when dealing with compound sentences, which is an exception to the comma rule: When the subject is the same for both independent clauses and is expressed only once, a comma is required if the connective is but. If the connective is and, the comma should be omitted if the relation between the two statements is close or immediate. Examples:
He has had several years’ experience and is thoroughly competent.
I have heard his arguments, but am still unconvinced.
Writers should avoid writing loose sentences that string together several independent clauses. Such constructions should be broken into separate sentences or separated with a semicolon.
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