What events that took place before the first act are causes for the present action?

The play opens with Nora's return from a Christmas shopping spree; he husband immediately chastizes her for spending money they don't have.

"Nora Helmer enters the house with packages and a Christmas tree. She pays the porter double what she owes him and eats some macaroons. Her husband, Torvald Helmer, comes out of his study and addresses Nora with tenderness and authority, calling her his “skylark” and his “squirrel.” Nora tells Torvald that she wants to show him what she has bought, and Torvald teases her for being a spendthrift. Nora replies that she and Torvald can afford to be extravagant, since Torvald’s new position at the bank means he will earn a large salary. Torvald replies that he will not take over that position until after the new year begins. When Nora argues that they can spend on credit until Torvald is paid, Torvald scolds her, reminding her that if something were to happen to make them unable to pay off their loan, they would be in trouble. He concludes by saying that he hates debts because “[a] home that depends on loans and debts is not beautiful because it is not free.”

http://www.sparknotes.com/lit/dollhouse/section1.html

The transaction between Nora and the porter that opens A Doll’s House immediately puts the spotlight on money, which emerges as one of the forces driving the play’s conflicts as it draws lines between genders, classes, and moral standards. Though Nora owes the porter fifty øre (a Norwegian unit of currency), she gives him twice that amount, presumably because she is infused with the holiday spirit. While Nora likes to spend and allows the idea of buying presents to block out financial concerns, Torvald holds a more pragmatic view of money, jokingly calling Nora a spendthrift and telling her that she is completely foolish when it comes to financial matters.

Torvald’s assertion that Nora’s lack of understanding of money matters is the result of her gender (“Nora, my Nora, that is just like a woman”) reveals his prejudiced viewpoint on gender roles. Torvald believes a wife’s role is to beautify the home, not only through proper management of domestic life but also through proper behavior and appearance. He quickly makes it known that appearances are very important to him, and that Nora is like an ornament or trophy that serves to beautify his home and his reputation.

Torvald’s insistence on calling Nora by affectionately diminutive names evokes her helplessness and her dependence on him. The only time that Torvald calls Nora by her actual name is when he is scolding her. When he is greeting or adoring her, however, he calls her by childish animal nicknames such as “my little skylark” and “my squirrel.” By placing her within such a system of names, Torvald not only asserts his power over Nora but also dehumanizes her to a degree. When he implies that Nora is comparable to the “little birds that like to fritter money,” Torvald suggests that Nora lacks some fundamental male ability to deal properly with financial matters. Though Torvald accuses Nora of being irresponsible with money, he gives her more in order to watch her happy reaction. This act shows that Torvald amuses himself by manipulating his wife’s feelings. Nora is like Torvald’s doll—she decorates his home and pleases him by being a dependent figure with whose emotions he can toy.

In addition to being something of a doll to Torvald, Nora is also like a child to him. He shows himself to be competing with Nora’s dead father for Nora’s loyalty. In a sense, by keeping Nora dependent upon and subservient to him, Torvald plays the role of Nora’s second father. He treats her like a child, doling out money to her and attempting to instruct her in the ways of the world. Nora’s gift selections—a sword and a horse for her male children and a doll for her daughter—show that she reinforces the stereotypical gender roles that hold her in subservience to Torvald. Nora sees her daughter the same way she has likely been treated all of her life—as a doll.