This post explains the concept of informal formal speech(존댓말) and speech(반말) used in Korean.
※ I kindly ask for your understanding if my English is somewhat lacking or awkward.
반말 & 존댓말
In Korean, it’s important to distinguish between 반말(informal speech), which is used among close friends or peers, and 존댓말 (honorific or formal speech), which is used when speaking to someone older or in a higher social position.
Except in certain settings where everyone is clearly of the same age—such as schools or academies—Koreans generally expect 존댓말 to be used when meeting someone for the first time.
If a Korean hears 반말 from someone they’ve just met, it can be considered extremely rude and, in some cases, may even lead to conflict.
※ If a foreigner speaks 반말 by mistake, most Koreans are generally understanding