In my opinion, the most formidable thing in learning a language is somehow adapting oneself to use the natural expressions of that language. Even if I were to memorize thousands of words, my ability to express myself in a foreign language would not sound like a native person would sound. He or she is accustomed to every single natural combination of words from a lifetime of usage, but we, as ESL students, need to memorize them all, and often without context!