“Jal meokgetseumnida” (잘 먹겠습니다) — not “bon appétit”
It is a thank-you disguised as a statement about yourself, and it is aimed at whoever is feeding you. Say it to a waiter and it lands wrong.
etiquette
“Sugohaesseoyo” — the everyday phrase that can still get you in trouble
The most-used sentence in Korean working life, and there is a live argument about whether you are allowed to say it to your boss. Both sides are real and it splits by age.