The period is a terminal mark. That means it marks the end of a sentence. It brings your reader to a full stop, gives the sentence a chance to sink in, and tells the reader the thought unit is complete. Effective use of the period requires knowing where your sentences begin and end.
Change: The room was empty. When we first got there. to: The room was empty when we first got there.
The first example above is split in two by the period, which separates the base clause from the dependent clause. The period falsely signals that the thought is complete, but it isn't. As a result, "When we first got there" is left alone, an orphan, a dependent clause with no base clause to depend on. It's a fragment punctuated as a complete sentence. Rejoining the fragment to its base clause solves the problem.
Effective use of the period requires knowing where your sentences begin and end.
Sometimes the opposite happens. The end of a sentence arrives, but instead of bringing the reader to a full stop, the writer plunges ahead with no break.
Change: It was a wonderful party we had a marvelous time. to: It was a wonderful party. We had a marvelous time. or: It was a wonderful party; we had a wonderful time. or: It was wonderful party, and we had a marvelous time.
Don't run independent clauses together without punctuation or splice them together with only a comma.
The first example fuses two base clauses into one sentence, causing a momentary double take. To make the meaning clear, we need to emphasize the thought divisions by punctuating the independent clauses as complete sentences, or by joining them with a semicolon, or with a comma and a coordinating conjunction. A comma alone is not enough.
Change: My legs were starting to weaken, my back was ready to give out. to: My legs were starting to weaken. My back was ready to give out. or: My legs were starting to weaken; my back was ready to give out. or: My legs were starting to weaken, and my back was ready to give out.
Don't run independent clauses together without punctuation or splice them together with only a comma. Join them solidly with a comma and coordinating conjunction, or separate them completely with a period. Likewise, don't split dependent elements off from their base clause. The word "dependent" means they can't stand alone as complete sentences. Essentially, if you know where every sentence begins and ends, you won't have trouble with periods.
4.20 Edit the following sentences for periods. In some cases you will find more than one possible version.
a. My father is a busy man. But a generous one. And a kind one.
b. I like going to the dog races when I get off work they really relax me.
c. If you knew what we had to put up with, you might show a little more sympathy, you might even try to help us. And not act so smug.
d. Whichever way you go. You will get there in about the same amount of time. A little more than an hour.
e. Little children must be taught to use caution. Crossing the street or walking out in a parking lot. They could get killed in a minute. Or maimed for life.