Getting Your Commas Right

In the course of editing the writing of several friends in the last couple years (and just reading random stuff on the internet), I have found a certain punctuation error that seems to be quite common.  And the English teacher in me, having a platform for instruction here, thought some of you might be interested in a short lesson.  (Because I have intelligent readers who care about such things.  Yes?)

So, here we go.  Suppose you want to combine the following two sentences:

He lay in bed all morning.
She read the newspaper to him.

Notice the comma that is necessary:

He lay in bed all morning and she read the newspaper to him. [WRONG]
He lay in bed all morning, and she read the newspaper to him.

When you are combining two stand-alone sentences (two independent clauses, each of which has its own subject - verb relationship), you need a comma before the "and" (or "but", "or", "so" . . . whichever conjunction you use).

Interestingly enough, the reverse error is also common.  Consider the combination of the following two sentences:

He lay in bed all morning.
He read the newspaper.

He lay in bed all morning, and he read the newspaper.

Most people would rightfully note that the second "he" is redundant and want to take it out to make it sound less awkward.  But note the punctuation change that must happen:

He lay in bed all morning, and read the newspaper. [WRONG]
He lay in bed all morning and read the newspaper.

Once you take out the second "he", you no longer have two independent clauses; you have one clause -- one subject with two verbs.  In this case, a comma is incorrect.  Usually, a writer will catch this in a simple sentence like the one above.  The error will happen when the sentence gets more complicated.

He lay all morning in his king-sized bed his parents had given him for a wedding gift twenty years earlier, and read the newspaper with his morning cup of joe. [WRONG]

So, here's the rule.  When combining two independent clauses (stand-alone sentences) with a coordinating conjunction (and, but, or, nor, so), you need a comma before the conjunction.  When you have one independent clause -- one subject with two verbs -- you do not need a comma before the conjunction. 

Clear as mud?  I thought so.  ;)  I hope you were taking notes; there will be a short quiz next period . . .