This is inspired by Glowing Fish
's Chinese words for 'yes' and 'no'
It's interesting that a commonly used word for yes in Chinese is the verb "to be," because "yes" and "no" in Korean are "ye" and "a-ni-yo," literally "(it) is," and "(it) isn't." This shouldn't surprise one too much, though, because Korean is related in some ways to Chinese, in that it borrows a lot of words, and used to be written with Chinese characters before the invention of Hangul by King Sejong the Great.
Another commonly used negation is "an-dwei," although the "w" there is barely pronounced, so it sounds more like "an-dae." This is the negative form of the verb stem "dwei-da," which means something like "to become." However, that verb is also used as a part of grammar to form sentences that have a meaning along the lines of "it's okay (if ... happens)." So, using by itself in the negative form carries a connotation akin to "No way!" in English. As in: "Alex! Nak-ji tang meok-ja!" "An-dwei!" ("Alex! Let's eat some octopus soup!" "No way!").
Both a-ni-yo and an-dwei make use of the negative prefix "an," which can be used with any verb or adjective. It's common to respond to a question by repeating the verb as a statement, positive or negative depending on the answer. "A-jik bap meo-geo-sseo?" "An meo-geo-sseo." ("Have you eaten dinner/lunch yet?" "Nope.")
Note: As always, the "eo" used in the standard Romanization of Hangul represents a kind of neutral vowel sound, between "eh" and "oh," like the o in "hot," not an "ee-oh" sound, like "Leo."