What is the lesson of the play "A Doll's House"?
I'm assuming that I can address the theme in response to your question. Ibsen wrote plays that dealt with taboo subject matter of the time, such as the place of women in society, venereal disease, marriage, double standards, etc; "A Doll's House" features all of these issues.
The play examines the marriage of Nora and Torvald Helmer, and lets us watch as Nora finally grows up and sees the world as it is. She faces the double standard of her inability to borrow money (even to save her husband's life), she faces the fragility of her position when being blackmailed, and she ultimately faces the inequality of her marriage.
One of the strongest moments in the play that expresses one of Ibsen's themes (lessons) comes when Torvald tells Nora that she shouldn't expect that a man to sacrifice his honor for the sake of his wife. Her response is that "millions of women have". Ibsen really was ahead of his time - to suggest in 1879 that marriage should be a partnership and that a woman had the right to place her own needs above others in order to be a complete person.
A Doll's House is a play about a woman, Nora, who struggles in a marriage that is controlling. Because her husband, Torvald, is controlling and treats her as a possession, Nora eventually leaves her husband and her children to start a new life and find herself. One of the themes of this play involves the role of a woman in a marriage and how that role defines them. Nora struggled with desiring to be a good wife and being true to herself and her own desires and needs. Her husband was clueless as to who Nora really was because he only saw her as a wife, not an individual. Nora could take no more of this and decided to leave her entire life behind.