Typically, the protagonist in a tragedy is ambitious, and tries to get ahead in some way that leads to tragic results. In Tess of the D'Urbervilles, it's her parents' foolish ambition that paves the way for the tragedy. Tess, as we all know, is raped by her "cousin," Alec D'Urberville. After the baby dies, Tess leaves home for a fresh start at the Talbothays Dairy.
Everything seems to be going fine for Tess at Talbothays – everyone's happy, no one knows her past, and she and Angel Clare are falling in love. She doesn't want to get married because of what happened to her with Alec. But she's so in love that she decides that the past doesn't matter, and finally agrees to marry Angel. Again, though, things aren't really in Tess's control – it was Angel who persuaded her to get married.
Marrying Angel without telling him about her past ahead of time was a big mistake, and it locks her into the tragic trajectory. Angel leaves her by herself, and her pride makes it impossible for her to ask his parents for money when she runs out. She has to work for herself and her family.
Alec just won't stop harassing Tess. He taunts her for being abandoned by her husband, and continually insists that he'll never come back to her. Then, after her father dies, he steps in and offers to help Tess's mother and younger siblings – but only if she'll marry him. Things are completely out of Tess's control at this point.
Once Angel gets back from Brazil and begs Tess for forgiveness, she realizes that Alec has completely ruined her life. His actions have separated her from Angel twice now, and his rape hurt not only her, but also Angel. Tess gets arrested and executed for murder.