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 independent clause has both a subject and a verb.
One of my clients failed to place a comma in this sentence:
The week of Christmas at the Washington Dulles International Airport was a madhouse and the traffic was much heavier than usual.
Where are the independent clauses in this sentence (I have underlined the subjects and verbs)? The writer should have place a comma after the word madhouse, before the conjunction and, 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.
Also keep in mind that you should avoid writing loose sentences that string together more than two independent clauses. Such constructions should be broken into separate sentences or separated with a semicolon.