V + 여/야 돼요 shows actions that you should/are supposed to do:
하루 세 번 식사를 먹어야 돼요.
You should eat (a meal) three times a day.
피아노를 잘 치면 매일 연주해야 돼요.
If you want to play the piano well you should practice every day.
건강 하려면 신선한 과일과 야채를 먹어야 돼요.
If you want to be healthy you should eat fresh fruit and vegetables.
V + (으)면 안 돼요 is the negative form:
이것은 만지면 안 돼요.
You shouldn’t touch it.
약을 먹어면 술을 취하면 안 돼요.
You shouldn’t drink alcohol if you’re taking medicine.
담배를 피우면 안 돼요.
You shouldn’t smoke cigarettes.