Sula and Nel become friends
The friendship between the two girls launches the rest of the events in the novel.
Sula sleeps with Nel's husband, Jude
Although other important events occur in their lives, this moments triggers the break in Sula and Nel's relationship, separating them for many years.
Sula fails to understand why her affair with Jude is wrong
Sula's guilt isn't clear-cut, since she can't understand feelings of jealousy associated with marriage. She simply remembers how things were with Nel—they shared everything as kids. Why not share Jude?
Nel finally confronts Sula
This is the moment we've been waiting for: Nel finally vents her anger and pain and asks for an explanation from Sula.
Sula's explanation of the affair
We, along with Nel, have been waiting to find out why Sula did what she did. But she fails to give a satisfactory answer.
This would seem to be the conclusion, but Sula's death represents a winding down of the story, not an end to it. There's still important stuff to come.
Nel finally cries for Sula
This is sort of a conclusion, but an uncertain one. We learn that Nel's cries can't be contained; they have no beginning and no end, so the conclusion is open to interpretation. It could be that Nel finally finds peace by grieving for Sula, or it could be that a whole new type of pain has just been unleashed.