One event that dramatically impacts Huck is when he gets caught in the crossfire of the battle between the Shepherdson and Grangerford family. His good friend Buck dies, and Huck is so traumatize that he hides in a tree until it is over, and has nightmares about it for a long time after that. He feels like he played a role in their battle, because he relayed messages that kind-of started it, and thinking that he might be somewhat responsible for Buck's death really bothers him. He writes, at the end of chapter 18,
"I ain't a-going to tell all that happened--it would make me sick again if I was to do that...I ain't ever going to get shut of them--lots of times I dream about them."
Huck, normally a pretty laid-back kid, is really bothered by the feud and the deaths, and refuses to even speak about it in detail--it was obviously a very traumatic event for him.
Other dramatic events occur as he struggles with his conscience. It bothers him that he his helping a runaway slave, and several times he tries to turn Jim in. One time on the river he paddles out to tell some guys that he has a slave, but he can't do it. His "cowardice" bothers him. Later, he chickens out on writing Miss Watson to tell her where Jim is. He is bothered by this also, and concludes he's probably going to go to jail for it. These events really bug him, and nag at him throughout the entire story.
Huck is also dramatically impacted by Jim's willingness to stay behind with the wounded Tom Sawyer while Huck gets a doctor. He realizes that Jim is willing to sacrifice his freedom to help a friend out, and that moves Huck. It moves him so much that he is finally able to, for the first time, see Jim as his equal, as "white on the inside," and as someone worth respect and admiration on equal standing.
I hope that those thoughts helped; good luck!