-어야 되다

V/Adj/이다,아니다 + 아/어야 되다(하다) (-어야(only when~) + 되다(it’s okay)) This indicates an obligation or necessity for something or a situation.(need to~, should~, have to~, must~). For this grammar pattern, '-되다' can also be expressed as '-하다', and both expressions have the same meaning.

내일까지 숙제를 해야 돼요.

I have to do my homework by tomorrow.

물을 하루에 2L 이상 마셔야 해요.

You have to drink more than 2L a day.

약속을 꼭 지켜야 해요.

You must keep your promises.

→ If you want to emphasize obligation more strongly, like the English expression "must~", you can simply add adverbs like 꼭(at any cost, tightly) or 반드시(no matter what, surely).

• 안 ~어야 되다 vs ~으면 안 되다

Both mean “you should not, you must not~”, but subtle nuance is different. To compare the exact meanings, let's literally translate each component as follows;

안 ~어야 되다	~으면 안 돼다
안 ~ 어야 되다 only if not ~, okay 술을 안 마셔야 돼요. Only if you don’t drink, that’s okay(good).	~ 으면 안 되다 if~, not okay 술을 마시면 안 돼요. If you drink, that’s not okay(good).
"안 ~어야 되다" is often used to emphasize that a certain action must not be done, especially when expressing determination or giving advice in contrast to an undesirable outcome.	On the other hand,"~으면 안 되다" is more naturally used to express rules or warnings in general.