Although Joan Durbeyfield is a minor character in the novel, how does Hardy use her as a tool to bring out larger themes of Tess of the D'Urbevilles?
Tess's mother, Joan, is a a superstitious, yet positive and strong woman who just wants the best for her daughter. She simply wants Tess to marry a nobleman and have a good life. Joan is supportive and loving towards Tess no matter what misfortune befalls her daughter. Joan represents the older and more experienced woman of the time period, but does not inform her daughter of the ways of the world or the ways of men--probably because such things were simply not discussed outwardly at the time. After Tess returns home the first time after being with Alec, and pregnant, she asks her mother why she never told her of the ways of men. Joan felt badly about that, but she didn't think that she would have to since noblemen were supposed to be responsible and kind according to their social standing. In this, Joan herself is also as naive as her daughter. Joan steps up to the plate later and advises her daughter not to tell Angel about what happened with Alec when she was a girl. She specifically writes to Tess and orders her not to reveal anything about the baby she had and who died. Sadly, we will never know if that advice would have saved Tess's marriage because Tess does reveal her past to Angel. So larger themes brought out through Joan would be motherly-love, motherly-commitment, a woman's typical life at that time in England, and a woman's perspective on male/female and marital relationships.