The equations of physics and geometry: classification into elliptic/parabolic/hyperbolic, the heat, wave, and Laplace equations, separation of variables, the method of characteristics, well-posedness, transform methods, boundary-value problems, and the CFL condition for numerical schemes.
Free to start · adaptive placement finds your level · reviews timed to your own forgetting.
Every idea is taught with motivation and a worked example before the drills, and an FSRS spaced-repetition engine schedules each review for the day its model predicts you would forget it. A short placement check finds what you already know, so you start Partial Differential Equations exactly where it's useful.