Some context: I was taught Haskell, Prolog and Java at university (2006-2010). Coming from chaotic JavaScript and PHP scripting all were hard to grok, but I enjoyed them all. After a couple of years only Java stuck in my working memory. So: I could program in Haskell, I've probably used monads and functors fluently, but the concepts still don't click. I have the same problem with grammatic concepts like adjectives, adverbs, and the more complicated stuff. I obviously use them all the time, but I'm not fluent in the theory.