What remains vivid for me are those subjects taught by teachers who loved their stuff. They were better at communicating the broad strokes of the subject matter; other teachers gave us the details but couldn't help us find the big picture or the personal connection with a subject. (It does make you question all the effort that goes into curriculum design, if it all comes down to the skill of the teacher).

The broad strokes remain somewhat sort of intact, even though the details are long gone from mind. Sometimes all that remains is a kind of mental placeholder... as if to say, "I don't remember how to (fill in the blank) but I know that knowledge exists and I can find it if I need it."

And sometimes there's not even that. Despite not-bad marks in all the senior maths, I have NO idea what it all meant.