Fixing Errors and Commas

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 not which if you want to restrict which book in the following examples:

 Restrictive: 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 yet), 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.