I think Tom felt sorry for Mayella so he help her and did not like her the way she did
I think Tom's mistake of feeling sorry for a poor, white girl is very unusual in the town. I think it also shows that not all black people are bad people and they don't hate white people.
Why is the book in two parts? Well, in the first part of the story, it talks about the Finch family and their background, and what the children's lives are like. Where as in the second part, it explains more in depth on the trial between the Ewell's and Mr. Tom Robinson. If it was not set up this way, you can get easily confused and lost in when the topic is occuring and what is going on in the story
The book is sectioned in two parts, because it shows how you open a door to the other parts and seperates the personal 
The book is divid into two parts because there is a time difference. It takes you into a sooner time.