I am teaching Calculus III this semester and a student signed up for this course after completed Calculus I and II in a different institution.
I quickly realised that this student does not understand many basic concepts from Calculus I and II. For example, she has never seen Polar Coordinates, does not understand Taylor's series, and cannot compute some simple integrals such as $\displaystyle\int e^{-x^2}x\mathrm{d}x$.
If she does not care about her grade in Calculus III and does not study for it, I probably will just give her an "F" and move on. But the reality is, she admits that she had a horrible Calculus teacher before and she did not learn much in her Calculus I and II class, but she got passing grades anyway and now she is in my Calculus III class. She works very hard and keeps talking with me and e-mailing me about her questions and difficulties. While I should be helpful to my students and I am trying to be, I also feel I am spending too much of my time on this single student.
When she seeks my help with concepts from Calculus I and II, I feel frustated as it seems I am paying the price for the incompetence of her previous calculus teacher. Should I tell her to retake, or at least sit in some Calculus I and II class? In general, is it a teacher's resonsibility to help a student with his/her problem from a prerequisite course?