Every teacher of calculus encounters various degrees of student understanding. To be a successful teacher, it is essential to understand student misconceptions and to make clear explanations to one's students. To better understand how new teachers, especially graduate students, develop their ability to understand student thinking, we conducted individual interviews with graduate students teaching calculus for the first time. We will present the results of our findings with the hope that this will lead to more effective teaching training programs for graduate students who will teach undergraduate mathematics.