I need some help on figuring out the main idea of "The Adventures of Huckleberry Finn." I know that for most of the part, Huck and Jim float on a raft down the Mississippi River and on the way, they encounter many people and obstacles. At the end of the novel, Finn is also more mature in his words and thoughts.

But, I am having trouble deciding on what the main idea really is and what Twain wants to show. Does it concern the society at that time?

Yes. This book certainly concerns society at that time.

Sparknotes should help you.

http://www.sparknotes.com/lit/huckfinn/

So, it seems like there are various ideas concerned with this book; slavery and how African-Americans were treated, how Huck eventually learns to think for himself, and how torture and abuse did not lead Huck on a "sivilized" path.

Right!

Understanding the main idea of a literary work can sometimes be challenging, but it's an important aspect of literary analysis. "The Adventures of Huckleberry Finn" by Mark Twain indeed explores various themes, including society, morality, and the journey of self-discovery. To determine the main idea, you can consider a few steps:

1. Read the novel: To begin, make sure you have a good understanding of the entire text. Sketching out the plot, noting significant events, and paying attention to character development will provide a broader context for identifying the main idea.

2. Identify recurring themes: Throughout the novel, note any recurring ideas, motifs, or conflicts. In "Huckleberry Finn," some recurring themes include race, freedom, morality, and hypocrisy. These can be helpful indicators of the main idea.

3. Consider author's perspective and historical context: To understand what Twain may be trying to convey, consider his own views and the historical context in which he wrote. Twain was critical of slavery, racial discrimination, and the hypocrisy of society, which may serve as important clues.

4. Analyze character development: Look closely at how the main character, Huck, evolves throughout the story. What lessons does he learn? What changes occur in his thoughts and actions? This transformation can offer insights into the main idea.

5. Evaluate author's message: Consider what Twain may be trying to communicate through the events, characters, and themes in the novel. What social, moral, or political commentary is he making? How does he challenge or expose societal norms or prejudices?

By following these steps, you can gather a clearer understanding of the main idea of "The Adventures of Huckleberry Finn" and what Twain intends to convey about society at that time. Remember that interpretations may vary to some extent, but with a careful analysis of the text, you can form a well-supported opinion on the main idea.