The comma is one of the most common and important punctuation marks in the English language. It is also one of the most commonly misused. It is not uncommon when asking someone why he or she has used a comma incorrectly to receive answers like ‘because it just looked like it needed one,’ or ‘I haven’t used one in a while and it is a really long sentence!’ When people respond in this manner, they reveal a fundamental lack of understanding of the comma and the way that it is used. Many different situations call for the use of the comma; however, the four situations that follow are possibly the most common and their mastery will give any piece of writing a degree of polish that is often the mark of a good writer.
To separate a subordinate clause from an independent clause
When a sentence consists of two clauses, it is important to consider the use of a comma carefully. If the first clause has been subordinated to the second by the use of a subordinating word or phrase like ‘although’ or ‘after’ or even ‘when’, a comma must be employed before the beginning of the independent clause. If the independent clause comes first, the comma may still be used but is not essential and may indeed be better left out. The same rule applies if the opening to the sentence falls short of a clause and is instead an introductory phrase; however, if that phrase is very short, the comma may be dropped.
To delineate between two independent clauses separated by a conjunction
The use, or non-use, of this comma is a common error. When two independent clauses are separated by a conjunction like ‘and’ or ‘but’, it is important to indicate this using a comma. This comma may be dropped if the two clauses are very short, but in longer sentences, it becomes important. However, a comma should never be used to separate independent clauses without the use of a conjunction. This is called a comma splice and will earn the ire of any marker. Try adding a conjunction or using a full stop or semi-colon instead of the comma.
To enclose parenthetic expressions
Parenthetic expressions are used to qualify, clarify or elaborate on the information given in a sentence. The most important thing to do when applying this principle is to consider which parts of your sentence are integral to its structure and which are not. The comma is used to enclose parenthetic expressions that the writer wants to be read there and then without undue interruption to the flow of the sentence.
To set off non-restrictive clauses
Though it sounds technical, this usage is fundamentally the same as the previous one. A non-restrictive clause often begins with ‘which’, but may also begin with words like ‘whose’, ‘who’ or ‘where’. These clauses come at the end of a sentence and are basically parenthetic, adding additional information and, as such, require a comma before them. In the case of the use of ‘which’, if you feel the information is in fact critical to the sentence structure drop the comma and use the restrictive clause ‘that’ instead.
Of course, there are many other instances where a comma is required. There is the serial comma for lists; the appositive comma for additional personal information; and the always-confusing delayed joining word comma. Like all things grammatical, these and all comma usages become easier with practise and become increasingly intuitive the more they are used. The important thing is to understand what the comma does and to save its use for that specific situation, and, in so doing, give the reader a clearer understanding of what you are trying to say.