Speech levels: 반말, 해요체, 합쇼체
Getting this wrong is the fastest way to sound rude
Three levels cover almost everything you need.
합쇼체 (합니다/하십시오) — formal, news, business, strangers in service settings. 해요체 (해요/하세요) — polite everyday. This is your default. 반말 (해/하자) — close friends, younger people, family.
Age and closeness decide, not the situation alone.
You use 해요체 with a stranger your own age, and 반말 with a close friend of the same age. The same person can be 해요체 on day one and 반말 six months later — usually after the older one suggests 말 놓자.
The dangerous mistake is going *down*, not up.
Being too polite is mildly stiff. Being too casual with someone older is a real offense. When unsure, stay in 해요체.