The comma was originally invented in order to avoid confusion in sentences; however, since it can be used in so many different ways, students are often confused as to when to use it and where it should be placed. Many students have been taught, for example, that a comma should be placed wherever the reader pauses. The problem with this reasoning is that each reader might pause at a different place in the sentence.
This section is intended to provide some instruction on the basic grammar rules that determine when and where the comma is commonly placed. These rules speak to the common errors that most students make when placing the comma. There are additional comma rules, but they are less troublesome for most students; therefore, they are not covered here.
The Six Most Commonly Misunderstood Comma Rules
Place a comma after an introductory or transitional word.
However, it is necessary to write a lot of papers in college courses.
Therefore, it is useful to know where to place the comma.
Place a comma after an introductory element (clause or phrase).
Given that he was unprepared for the test, he did perform well
At the end of the movie, we realized who the murderer was.
Place a comma after a dependent (or subordinate) clause. This is a specific type of introductory element.
Since it was raining, I brought an umbrella.
Because I didn't sleep well last night, I am tired today.
Place a comma before a coordinate conjunction in a compound sentence.
He was late arriving to class, and the other students had already
begun the test.
She went to bed early, so she would not be tired the next day.
HOT GRAMMAR TIP
If the clause in the middle of a sentence is restrictive (the meaning of the sentence would change if the clause was deleted), then no commas are used to set off the clause.
Place a comma to set off an appositive, an explanatory phrase after a noun. See the exception to this rule in the hot grammar tip.
Mrs. Smith, our next door neighbor, walks her dog every day.
Mr. Jones, the pharmacist at the drugstore, plays tennis every Friday.
Place a comma before and after a parenthetical expression.
We can go to the party, I suppose, if we don't stay too late.
There is a lesson here, I think, for all of us.