what does the narrator learn about Rebecca

Really she learns everything in bits and pieces. Initially she feels insecure about taking the place of this woman but it isn't until Maxim tells her everything at the end of the story that all the pieces fit together. There are quite a few pieces so I'd ask you to check out the GradeSaver short summary and you will see them all,

http://www.gradesaver.com/rebecca/study-guide/short-summary/