Where commas are placed can sometimes interfere with or aid in rhetorical effectiveness. Here are some of the most frequent uses of commas.
Commas after Introductory Element:
Subordinate clauses or prepositional phrases are often used at the start of a sentence to tell where, when, how, or why the main action of a sentence happens. Put a comma after an introductory element to signal the start of the main sentence.
- Ambiguous: When Joe sat down to eat his cat jumped on the table.
- Clearer: When Joe sat down to eat, his cat jumped on the table.
You could also revise the sentence by putting the subordinate clause at the end: Joe’s cat jumped on the table when he sat down to eat.
Unnecessary Commas:
Avoid putting a comma between grammatical elements that are logically connected: a subject and its predicate, a verb and its direct object, a preposition and its object, or an adjective and the word or phrase it modifies.
- Ambiguous: An old woman sitting on a park bench, tossed crumbs to the pigeons.
- Clearer: An old woman sitting on a park bench tossed crumbs to the pigeons.
Although a comma and a coordinating conjunction (and, but, or,…) can be used to form a compound sentence (e.g., I called my mom, and I asked how she was doing.), you do not need to put a comma before every conjunction:
- Ambiguous: I found the book in my room, and returned it to the library.
- Clearer: I found the book in my room and returned it to the library.
Here’s the rule: When two independent clauses are joined by a conjunction (fanboys), place a comma before the conjunction:
I deeply read all the assignments, so I was able to fully participate in the class discussion.
Here’s the other rule: When the conjunction is combining two verbs that are not independent clauses, do not include a comma:
I deeply read all the assignments and was able to fully participate in the class discussion.
Commas to set off a non-restrictive clause have a different meaning than those where no comma is used. If the clause is restricting the noun it is modifying, do not use a comma:
- Non-restrictive: Students, who have jobs, often learn to manage their time carefully. (all students have jobs)
- Restrictive: Students who have jobs often learn to manage their time carefully. (only the students who have jobs learn to manage their time)
More on Non-Restrictive Clauses:
A non-restrictive clause is not essential to the meaning of the sentence; it can be removed from the sentence without changing its basic meaning. Non-restrictive clauses are generally set apart from the rest of the sentence by a comma or a pair of commas. Use that n´Ç³Ù which if you want to restrict which book in the following examples:
-  R±ð²õ³Ù°ù¾±³¦³Ù¾±±¹±ð: Her book that includes seven stories means a lot to me. (This sentence is referring to the one book of all of her books that includes seven stories.)
- Non-Restrictive: Her book, which includes seven stories, means a lot to me. (This sentence is referring to the one book that we all know about. Incidentally, it includes seven stories.)
Missing Comma in a Compound Sentence:
When you join two independent clauses with a coordinating conjunction (and, or, nor, but, for, so, or y±ð³Ù), put a comma before the conjunction:
- Missing a comma: He wanted to travel but he did not want to go far from home.
- Revised: He wanted to travel, but he did not want to go far from home.
Comma Splice:
A comma splice is a run-on sentence that occurs when two independent clauses are joined by just a comma. Comma splices are considered unconventional in most academic conventions.
- Comma splice: The film told the story of a haunted house, it featured a frightening ghost.
- Replace comma with a semicolon: The film told the story of a haunted house; it featured a frightening ghost.
- Fix it by adding a comma and a coordinating conjunction: The film told the story of a haunted house, and it featured a frightening ghost.
- Replace comma with period and create two sentences: The film told the story of a haunted house. It featured a frightening ghost.