They caught him in a lie about where he came from. One man recognized Huck from coming down the river on the raft and canoe. They also used the handwriting differences. Last they were all asked what was tattooed on the man's chest.
2. Why is Huck upset when Jim is sold?
Because Huck and Jim were friends and he realized he would be better off with JIm’s friendship rather then the money he would get for selling him.
3. Why it is important that Huck says, "All right, then, I'll go to hell"?
Huck wasn’t bother by lying for most of the book. When it came to this part though he questioned himself. “What if lying is really bad” “Liars go to hell”, but then Huck remembered everything Jim had done for