Casual speech: -아/어 and the plain -다

Casual speech, 반말, is the polite form without its 요: 가, 먹어, 해, 봐, 마셔; in the past 갔어, 먹었어. "To be" becomes 이야 after a consonant and 야 after a vowel (학생이야, 친구야), and "not to be" 아니야. A question is the same form with a rising voice: 밥 먹었어? Yes is 응, no is 아니, and the pronouns change too: 나 and 내가for I너for you

Next to it stands the plain style, built on the dictionary form: verbs take -는다 after a consonant and a final n plus 다 after a vowel (먹는다, 간다), adjectives keep -다 (좋다), the past is 갔다. Its questions end in -니 (가니?, 먹니?), its commands in -아라/어라 (먹어라) and its suggestions in -자. Parents use it to children, older people to much younger ones, and everybody uses it in diaries and in exclamations: 와, 예쁘다!

Casual speech is for friends of about the same age who have agreed to it, for younger siblings, for children and often between partners. Age counts even by a year or two, and the agreement is usually made out loud: the older person suggests dropping the polite ending. Until then, adults who meet as adults stay with -아요/어요, however friendly they are.

The risk is real. Casual speech to a stranger, a shop assistant, a colleague or anyone older is rude, whatever the words mean and however kind the tone. And once a sentence is casual, everything in it has to be: no 저 or 제가, no 네 for yes. English speakers tend to drop the 요 because it feels like a mere shortening, or to mix a humble 저 into a casual sentence.