While making demands or giving polite directions is an important language skill, expressing an obligation or a necessity is equally important. The most basic way to express either an obligation or a necessity is using the Korean grammar expression -아/어야 하다 or -아/어야 되다. This little pattern will suit are your obligation or necessity needs. Let’s take a look.
You can consider the two patterns 아/어야 하다 or -아/어야 되다 as equals as there is actually no difference in meaning between them. You can use them interchangeably if you’ like. These Korean grammar expressions inform the listener that either is something obligated or that something is a necessity. This can be either a necessary action or a condition that has to be met.
가: 내일 추울 것 같아요.
나: 네, 그레서 따뜻한 옷을 입어야 할 거예요.
A: It seems tomorrow it will be cold.
B: Yes, so we’ll have to wear warm clothes.
How To Use
At first the pattern -아/어야 하다/되다 might seem very complicated. However, as always by just following the basic rules of Korean grammar it is as easy as taking candy from a baby. The basic rule you need to follow is the ‘last vowel’ rule. If the vowel of the verb stem’s last syllable is either ㅏ or ㅗ, you use -아야 하다/되다. However, if it neither of those two, you use the ending -어야 하다/되다. Of course, when it is a 하다 verb, the ending becomes 해야 하다/되다.
When it comes to negatives, the pattern -아/어야 하다/되다 is a bit unique as it has two distinct negatives. These negatives are distinctly different. The first potential negative you can use is -지 않아도 되다. You use this if you want to express something is not necessary or obligated. However, there is also you can also use -(으)면 안 되다. This pattern expresses that certain things are not allowed or, in other words, it expresses the necessity to not do something. Please refer to the individual articles on either -지 않아도 되다 or -(으)면 안 되다 for a more in-depth explanation.
PS: The pattern -아/어야 하다/되다 is derived from the Korean conditional pattern -아/어야, which is used to express a certain condition is required in order to realize something.