Being a teaching assistant (sometimes called a tutor or class assistant) puts you in an interesting position. While some professors approach it as a practicum, an opportunity for you to learn and exercise your pedagogical style, others use you to get their own work done. Your power is provisional and contingent. The expectations vary from course to course; the subject matter may be more or less familiar, making your job that much harder. The grading you do must accord with not only your own sensibility, but even more so with what the professor wants. You may have to run discussion sections only, or you may be expected to lecture several times.