Dan Ebberts's MotionScript is the most rigorous reference for After Effects expressions on the internet. Where most tutorials show you what to copy and paste, Ebberts explains the underlying logic — how expressions work, why they behave as they do, and how to think through problems from first principles.
The site has been active since the early 2000s and remains the authoritative source for expression writing in After Effects. Essential for any student who wants to move beyond presets and templates into genuinely systematic motion design.
Not a beginner resource — but once you understand the basics of keyframes and timing, this is the next step toward working programmatically.