불 (不)

Hanja to the rescue! 불- (한자: 不) is a nice little guy who fills in the prefix role of -un, -dis, -ir, -non, and the like. Slip him in front of many hanja based words and you've got the opposite. Negation at it's easiest. Examples:

가능하다 to be possible
불가능하다 to be impossible
예) 회의가 불가능할 것 같아요
예) 회의를 못 할 것 같아요.
Ex) I can't make it to the meeting.

규칙 (명사) rule, 규칙적인 (형용사) regular
불규칙적인 (adjective) irregular
예) "만들다"는 불규칙 동사이지요?
Ex) Isn't "만들다" an irregular verb?

합격하다 to pass a test
불합격하다 to fail a test
예) 중간고사를 못 봤어요. 불합격할 테니까 이제 수업을그만둘거에요.
Ex) I bombed the midterm. Since I'm pretty sure I'm going to fail, I'm just going to drop (quit) the class.

편하다 to be comfortable
불편하다 to be uncomfortable
예) 이 옷은 불편해요.
Ex) These clothes are uncomfortable.

법적인 legal
불법적인 illegal
예) 영화나 노래를 인터넷에서 다운로드 하는 것은 불법이에요.
Ex) Downloading movies or music off of the internet is illegal.



The only exception to this, as far as I can tell, is that if 불 is placed in front of words that start with aㄷ or ㅈ then it changes to 부 as in:

도덕하다 moral
부도덕하다 immoral
예) 뇌물을 받는 것은 부도덕한 일이예요
Ex) It's immoral to take part in bribes.

정확하다 accurate
부정확하다 inaccurate
예) 그 퍼스터에있는 정보는 부정확해요.
Ex) The information on that poster is inaccurate.

예) 내 발음이 부정확한가요?
Ex) Is my pronunciation inaccurate?

For more negation goodness, check out the page on the Korean Wiki Project.