불가능해요 · 불가능했어요 · 불가능한 (+noun)
impossible
Example
혼자서 이 일을 하루에 끝내는 것은 불가능해요.
It's impossible to finish this work in one day by yourself.
Word parts
불 not+가 can+능 able
불- = 'not', a prefix you'll reuse often: 불편, 불안, 불가능.
Good to know
Before ㄷ or ㅈ this prefix becomes 부-, as in 부족 'lack' and 부당 'unfair'.