Must, may, must not: -아/어야 해요, -아/어도 돼요, -(으)면 안 돼요

Have to

가야 해요 — I have to go. 먹어야 해요 — I have to eat. 해야 해요 — I have to do it.

Start from the 해요 form you already build. Remove 요 and add 야 해요: 가요 → 가 → 가야 해요; 먹어요 → 먹어야 해요; 해요 → 해야 해요. The vowel choice was made when you built the 해요 form, so there is nothing new to decide. It is written as two words.

-아/어야 해요 at school

숙제 is homework, 시험 an exam. 숙제를 해야 해요 — I have to do my homework. 내일 시험이 있어서 공부해야 해요 — I have an exam tomorrow, so I have to study.

The second sentence uses the 서 of the last unit: reason first, duty last. Textbooks write the pattern -아/어야 해요. Like English “have to”, it covers anything from a rule to a plain necessity, and no person needs to be named.

숙제 is pronounced [숙쩨].

May I? -아/어도 돼요

여기 앉아도 돼요? — may I sit here? 네, 앉아도 돼요 — yes, you may. 사진을 찍어도 돼요? — may I take a photo? 네, 돼요.

The base is the same: 해요 form minus 요, then 도 돼요. 앉아요 → 앉아도, 찍어요 → 찍어도. And 돼요 is the word from 카드 돼요?: “it works, it’s OK”. That is why the short answer 네, 돼요 is enough. The pattern is written -아/어도 돼요.

Must not: -(으)면 안 돼요

들어가요 (들어가다) is “go in”. At a museum door: 들어가도 돼요? — 아니요, 들어가면 안 돼요. No, you can’t go in. 여기에서 사진을 찍으면 안 돼요 — you mustn’t take photos here.

This one is built differently. It goes on the dictionary stem, not on the 해요 form: 들어가다 → 들어가면, and after a final consonant with 으: 찍다 → 찍으면. Then comes 안 돼요, “it is not OK”. The pattern is written -(으)면 안 돼요.

들어가요 is pronounced [드러가요].

Tickets, passports, bookings, smoking

표 is a ticket: 표를 사야 해요. 여권 is a passport: 여권이 있어야 해요 — you need a passport, literally “there has to be a passport”. 예약해요 (예약하다) is “reserve”: 식당을 예약해야 해요.

담배 is a cigarette and 피워요 (피우다) is “smoke”. Together, on a sign you will see often: 여기에서 담배를 피우면 안 돼요 — you mustn’t smoke here.

여권 is pronounced [여꿘]; 예약해요 is [예야캐요].

The three side by side

With 가다: 가야 해요 — must go. 가도 돼요 — may go. 가면 안 돼요 — must not go.

The first two sit on 가, which is 가요 without 요. The third sits on the dictionary stem plus 면. With 가다 you cannot see the difference; with 찍다 you can: 찍어야 해요, 찍어도 돼요, but 찍으면 안 돼요.