What is the major problem/conflict in To Kill a Mockingbird?

It's not really possible to pin down a single conflict in
Harper Lee's novel, To Kill a Mockingbird. There are actually two
major plots: Part One of the novel focuses on Jem's and Scout's fascination with their
unseen neighbor, Boo Radley. The primary conflict in this section is the children's fear
of Boo vs. their unquenchable urge to get a look at him. The children eventually come to
realize that Boo is friendly enough, though the section ends with Boo still being
invisible to them.

Part Two of the novel deals with the
trial of Tom Robinson. There are many conflicts concerning the trial: Atticus' decision
to defend a black man accused of raping a white woman is the major one. Whether Tom will
receive a fair trial because he is black is another. The family's standing in the town
is compromised because of Atticus' decision, and eventually, all of the Finches come to
realize that Bob Ewell's threats following the trial may be for
real.

The final conflict of the novel arises when the two
plots are tied together in the end. When Bob Ewell, who had made the false charges
against Tom, attacks Jem and Scout on a dark Halloween night, it is Boo who emerges to
save them, killing Bob in the process. Though it is never discussed, Boo's decision to
protect the children must have been a trying one; Sheriff Tate then has to resolve the
conflict with a decision of his own.