Languages are not math. In math one learns the basics, 1+1=2, which is used to teach the next step up X+1=2, and so on and so forth. There are constants, and constants are always true. 1+1 can never equal 5. You can't divide be zero. n1Sin(theta1) always equals N2Sin(theta2). Eventually you get into some fun things like i, positive or negative zeros, and how to deal with division by zero. But the rules never change. You just keep building your knowledge base with the tools you have.
In a language you learn a bunch of vocabulary, how say your name, age, where you're from. Then you learn to ask a very basic question. Soon you learn how to use new verbs. Then everything goes to pieces. Every language has some irregular verbs. But you can learn and master those verbs, you can even master the weird grammar. But sooner or later you pass from doing grammar drills and memorizing words into how to use the language. Sure, you can write a report, understand instructions from the teacher, and talk to friends, but the very nature of a language is that is spoken and it changes.
Ever tried talking in Olde English (Old English, not middle). You would sound like a drunk German-Italian hybrid and no-one would understand you. No matter how hard you study a language there will always be bits of it in which the nuances escape you.
For me, it's あいづち(aitzuchi). What is this funky word you ask? Simply put, its interjections.
In truth it's hell.
It is basically how to show you're listening to the speaker and understand. But it's not eye contact, and it's not nodding (although that is one form). It's actually phrases, repetitions, and words. I.e. speaker says something, and you say 'oooh', or 'aaah'. But the Japanese use it for everything and have tons of them. Tons. AND some are only used by males, or females, or have a more formal feeling.
I can memorize them, learn them, know their meanings and try to use them, but what I hate about them is that I'm essentially interrupting to say I'm listening. It seems so oximoronic that it drives me insane. That and in a few months time when I return the giant land mass I call home, I will seem like a freak for using the ones I do use.
Next time I learn a language, I learn one that doesn't involve me constantly interrupting someone.