Conventions of Composition Rule 2
Rule: Two independent clauses not joined by for, and, nor, but, or, yet, or so need to be separated by a semicolon. If you join then with only a comma, you've created an error called a comma splice.
Wrong: Quinn went to the beach one day to see his friend, his friend forgot to meet him there.
Better: Quinn went to the beach one day to see his friend; his friend forgot to meet him.
Also better: Although Quinn went to the beach one day to see his friend, his friend forgot to meet him. [Note, this version subordinates the first clause, so it no longer has two independent clauses.]
Wrong: Morgan is happy, she just won the prize.
Better: Morgan is happy; she just won the prize. OR Morgan is happy because she just won the prize.
Wrong: I wanted to go, my mom wouldn’t let me.
Better: I wanted to go; my mom wouldn't let me. OR I wanted to go, but my mom wouldn’t let me.
Wrong: Our llama has a sore hoof she stepped on a sharp rock that bruised it.
Better: Our llama has a sore hoof because she stepped on a sharp rock that bruised it.
Also better: Our llama has a sore hoof; she stepped on a sharp rock that bruised it.
Practice revising the sentences below, deciding if and where they need semicolons, commas, or conjunctions:
The murder in the tunnel created a nightmarish traffic jam nevertheless drivers waited patiently for more than an hour.
For many years, the Wizard of Oz was the all-time favorite movie at one time, every kid on my block wore ruby slippers.
When the tiny babies started to appear around campus, everyone loved finding them, nobody knew who kept putting them up.
My daughter said we needed another dog to keep us company, we already have six dogs.
She gave good advice, she told us all to look up and look around.
Resources for more explanation of how to avoid comma splices by using semicolons:
University of Wisconsin-Madison's Using Semicolons
Georgia College's Punctuation Rules (scroll down)