The fastest way to actually understand anything

Passive review, reading a paragraph again, watching a video a second time, can make an idea feel familiar well before it is actually usable. Familiarity and understanding feel similar from the inside, which is exactly what makes this trap easy to fall into. Explaining an idea from memory, in plain words, to an imagined beginner, exposes the exact place your understanding gets blurry, because you cannot fake a plain explanation the way you can fake recognition. The moment you get stuck, unsure how to phrase the next part, is not a failure. That stuck point is the single most useful thing to go learn next, since it marks the actual edge of what you understand rather than what you merely recognize. This is sometimes called the Feynman technique: explain it simply, find where it breaks, go fix that part.