It is + [필수/필요/중요/의무/당위/긴급/이상 형용사] + that + S + (should) + do
★ that절 안에 should가 생략되어 동사원형을 고르는 것이 출제 포인트
[필요] essential, necessary
[중요] important, vital, critical, crucial
[의무] imperative, mandatory, obligatory
[당위] no wonder, desirable, natural, proper, reasonable, advisable,
[긴급] urgent
[이상] curious, strange
