Me too: it took until 6 months after I started differential calculus (AS-level Pure 1 (UK)) before I grokked what the "d" in dy/dx meant (just like this person: https://physics.stackexchange.com/questions/65724/difference... ) - before then I was treating the differentiation operator as a magic, irreducible, symbol and only getting so far in the course because I was remembering and applying the mechanics of differentiation (i.e. by-rote) instead of really understanding what I was doing.