The answers are:
Madame Loisel’s actions propel the story forward.
Madame Forestier’s revelation about the necklace resolves the plot.
In the short story "The Necklace," by Guy de Mauppassant, Madame Loisel's disatisfaction and longing for wealth, possessions and attention set off the narrative. If she had refused to go to the party or had wore simpler clothes without any jewellery, she wouldn't have suffered privations. On the other hand, after so many years of hardship, Madame Forestier's confession that the necklace is fake finally determines the Loisel's misfortune.