[wiwi]

Thanks. I know that already, though.
The problem is: according to my limited knowledge from Minna no nihongo textbooks, there is no "ku arimasen" (maybe higher level?) but we are taught to use "kunai desu" as formal form. One of my friends, very good at Japanese, told me that both "kunai desu" and "kunai" sounds childish and improper. I don't know if it's right or not and what does the Japanese often use in what situations?