1. Who is the protagonist? Who is the antagonist?

It's difficult to tell who the protagonist is since the main character, Eckels, isn't exactly a good character.
I would have to say he's more the antagonist since his actions end up altering all of time for the worse, and I'd say the guide Travis is the protagonist.

2. How does the point of view from which the story is told affect your understanding of it?

It's told in the limited third person, which lets you not feel too attached to any of the characters since you can't hear their thoughts. It also lets you see more and know more than you might in the first person.

3. Why are the time travelers told to "stay on the Path?" What is "the path"? What could happen if a traveler goes off the path or steps on a mouse, for example?

They are told to stay on the Path so they don't alter anything in the past and thereby alter the future. The Path is an anti-gravity metal, allowing the path to hover several feet above the ground. It is intended to prevent people from the future from touching things from the past. If a traveler were to go off the path and kill a mouse, all of that mouse's descendants would die and anything that fed on that mouse and its descendants would die and so on, destroying possibly entire species.

4. The narrator uses the phrase "A sound of thunder," several times in this story. What is the sound of thunder being referenced?

Thunder refers to loud noises, rumbling, sometimes from guns. At