The most common Korean word for “when” is 때 (ttae). There is one more word with a similar meaning — compare them below. 「when」는 한국어로 때.
time; moment
TOPIK 1 · bound noun
when; what time
TOPIK 1 · pronoun
Words and levels: TOPIK I essentials (National Institute of Korean Language standard curriculum, KOGL Type 1) · examples: Cheesepotato