The following quote sums up why teaching and learning English as a Second or Other Language is so difficult.
"But perhaps the single most notable characteristic of English--for better and worse--is its deceptive complexity. Nothing in English is ever quite what it seems. [...] As native speakers, we seldom stop to think just how complicated and illogical English is. Every day we use countless words and expressions without thinking about them--often without the faintest idea of what they really describe or signify. What, for instance, is the hem in hem and haw, the shrift in short shrift, the fell in one fell swoop? When you are overwhelmed, where is the whelm that you are over, and what exactly does it look like? And why, come to that, can we be overwhelmed or underwhelmed, but not semiwhelmed or--if our feelings are less pronounced--just whelmed? Why do we say colonel as if it had an r in it? Why do we spell four with a u and forty without?"
Seriously though, I think about the English language on a more complex level everyday now at work than I ever did as a formal English minor in college. Now, isn't that ironic?