The following are very common mistakes related to punctuation usage (or lack of usage).
A comma splice results when a comma is incorrectly used to join two complete thoughts (meaning each thought has a subject and verb, and the thoughts could stand all on their own—otherwise known as “independent clauses”). Think of it this way: a comma is a weak punctuation mark, and you need something a little stronger to separate two complete thoughts (such as a period or a semicolon—see below for more solutions).
Comma Splice: I had class at 9a.m., I woke up at noon.
A run-on sentence (sometimes called a “fused sentence”) results when two complete thoughts (a.k.a. “independent clauses”) are joined without any punctuation separating them. Hence, your sentence just runs on and on…
Run-On Sentence: I had class early this morning I woke up at noon.
See below for five solutions to your run-on sentence dilemma.
I had class early this morning. I woke up at noon.
I had class early this morning, but I woke up at noon.
Side Note: A simple way to remember coordinating conjunctions is to use the acronym FANBOYS (for, and, nor, but, or, yet, so).
I had class early this morning; I woke up at noon.
I had class early this morning; however, I woke up at noon.
Although I had class early this morning, I woke up at noon.
Fragments results when an incomplete thought is punctuated as if it were complete sentence.
Sentence Fragment: Down by the bay.
That looks like a complete sentence because it begins with a capital letter and ends with a period, but it is not. It begs the question, what is down by the bay?
Fragments can happen for a lot reasons (maybe there is no subject, maybe there is no verb, or maybe you’ve written a modifying phrase that isn’t actually modifying anything). The previous example could be edited many different ways to achieve a complete sentence, but here’s one example:
Correction: The blue heron landed down by the bay.
Now I have both a subject (“blue heron”) and a verb (“landed”).