Huck learns responsibility:
He is immature at first, playing practical jokes on Jim, treating him the way Tom Sawyer would--as an inferior toy. However, Jim teaches him to be responsible and to care about other people's feelings. Recall the rattle snake incident when Jim got bitten by a dead snake's mate. Recall the fog incident when Jim was worried sick whether or not Huck was all right, but he thought it was hilarious to lie to him that it was just a dream. For the first time he learns to apologize to a slave, and that a slave is just like anyone else and deserves respect.
Huck learns about love:
His father was an abusive drunk. He did not teach Huck any good values, did not allow him to get an education, taught him how to steal, neglected him, and regularly and brutally abused him. Huck had to stage an elaborate plot of his own death just to escape from Pap's abuse.
However, Jim teaches what it is like to be loved. Each night he keeps Huck's watch and lets Huck sleep, he calls him "honey" and is always nice to him. He teaches him values of respect, friendship, and loyalty. For the first time, Huck has a father figure who shows him what love feels like. He grows emotionally by developing a bond and care with someone.
Huck learns morals--what is right and wrong:
This is the major inner conflict. All along he has been torn by the decision whether or not to turn Jim in to the slave catchers. At first, he saves Jim because he needs him. Huck is lonely and prefers to have company when he runs away from Pap. Later on, he lies to the slave catchers that his family has the small pox on the raft and deters them, but he says that whether he tells the truth or lies, either way he would have felt terrible, so he'll do whatever comes easier. This is a child talking. Finally, in the end, when he tears up the letter to Miss Watson telling her where Jim was, he DECIDES to damn his soul to hell in order to protect Jim. This is a much more mature decision. He has developed a love for Jim, and consciously decides to go against the teachings of his society and PAY A PRICE for it--go to hell. His motive this time is not a selfish one, but a selfless one--for his friend Jim.