It sounds like it's time you reread the novel tonight, but if you don't have the time, you can at least review the eNotes chapter-by-chapter summaries found at the link below.
As for the most important chapters, I would reread those that include the trial of Tom Robinson (Chapters 16-21). Other important events can be found in Chapter 15, when the lynch mob comes to try and take Tom; Chapter 10, the scene with the mad dog; and Chapter 2, Scout's first day at school.
The climactic scene comes in Chapter 28, when the children are attacked by Bob Ewell. The final three chapters are also important, since Boo Radley makes his only appearance. The scene with Burris Ewell, which you asked about, appears in Chapter 3.
Good luck with your exam--you may need it if you only decide to review what are considered the main chapters. It is a novel that should be read from beginning to end to fully understand and appreciate it.