Have a look at the following sentence:
The day is finally over and I am ready for bed.
Notice anything wrong with it? Here we have two independent clauses joined by a coordinating conjunction. Ah, you’re telling me that the problem is a proliferation of grammatical terms. Let’s dispose of coordinating conjunctions first. The list is simple: and, but, or, nor, for, so, yet. There, that was easy, just those seven words. Now what do they do?
Join sentences, that’s what. “The day is finally over,” could stand by itself, as could “I am ready for bed.” An independent clause is something that could be a sentence if it were by itself. In the first clause, “day” is the noun whose condition is asserted by “is.” The same happens with “I” and “am” in the second clause. Since in each case we have a subject and a verb that agree, there are two independent clauses joined by “and.” This means that there must be a comma after “over.”
That sentence had state of being verbs. Now let’s try some action:
The clock chimed, and I shook from fright.
What chimed? The clock. Who shook? I did. Again, we have subjects and verbs in agreement, so “The clock chimed” and “I shook from fright” are independent clauses. They could each stand alone. That’s the reason for the comma after “chimed.”
Now if you recall the previous lesson on semicolons, you’ll realize that the sentence could have been written, “The clock chimed; I shook from fright.” I prefer it with the comma and conjunction, though. The second half is the result of the first, not just a coincidental event.
How about this one?
The dog ran over and barked.
Notice that there’s no comma before “and” in that sentence. Why not? “Dog” is the subject of both “ran” and “barked,” so no comma goes in between. Do not separate subjects from their verbs, except in cases like this:
The dog ran over, crossing a rushing stream, and barked.
In that case, “crossing a rushing stream” could be left out of the sentence, so it’s separated off by commas before and after it.
Everything make sense now? Try this, if you’re ready:
Go to sleep and dream of me.
That one is also wrong. The rule is that two independent clauses joined by a conjunction must have a comma in between. “Go to sleep” and “dream of me” are just such clauses. The verbs, “go” and “dream,” do have subjects, even though they aren’t stated. The implied subject is “you.”
Perhaps that’s enough for now, as I don’t want to make my readers, um, dare I write it, commatose.