언제
eonje · TOPIK 1
when
언제 = 어느 (which) + 적 (time) + 에 (at): at which time
Historically a contraction of 어느 적에, literally at which time, squeezed down to 언제. The 언 is the same which-root found in 어느 (which) and 어디 (where), so the question words form one family.