Awhile back, I tried to teach myself Spanish. I got some used primers and some notebooks, and away I went. After a few weeks, I tried to converse with a friend of mine who was fluent. It didn't go well. I would hear the words she spoke and get this nagging feeling of familiarity, but true understanding evaded me. As I went back to the drawing board, I realized that as I studied alone, I tended to do it silently. Instead of processing the words as sound and phonemes, I was processing text and symbols. Subsequently, I could read and write, but not speak or listen.
As I study calculus, I am reminded of this past mistake and am taking pains not to repeat it. I speak the words aloud as I work through a problem, rather than simply processing symbols and manipulating formulas. I get the same answers, but I gain more understanding.