Must (classical): 〜べし

べし is the classical auxiliary of obligation: “must, one shall”. You meet it in mottoes, maxims, signs and solemn texts (学生は勉強すべし). Its family is very much alive in modern Japanese: べき + noun, べく (“in order to”), べからず (“it is forbidden to”). Classical, solemn register.

• 学生はよく学び、よく遊ぶべし。がくせいはよくまなび、よくあそぶべし。Students shall study hard and play hard.
• 関係者以外立ち入るべからず。かんけいしゃいがいたちいるべからず。Authorised personnel only.
• 初心忘るべからず。しょしんわするべからず。Never forget the beginner's spirit. (a maxim of Zeami)

べし comes from classical Japanese, where it expressed obligation and necessity, but also strong probability or appropriateness. In modern Japanese it's used on its own in mottoes, old regulations and formulas meant to sound weighty — rather like English “thou shalt” or “shall”.

Above all, its inflected forms are everywhere: べきだ (should), べき + noun, べく (in order to), べからず (forbidden), べくもない, べくして. Knowing the family helps you read all formal writing.

• 鉄は熱いうちに打つべし。てつはあついうちにうつべし。Strike while the iron is hot.

べし attaches to the dictionary form. する gives すべし (or するべし). The forms below are all alive in modern written Japanese.

• 水は人間にとって欠くべからざるものだ。みずはにんげんにとってかくべからざるものだ。Water is something indispensable to human beings.
• 来るべき未来に備えて、今から準備しておこう。きたるべきみらいにそなえて、いまからじゅんびしておこう。Let's start preparing now for the future that lies ahead.

〜なければならない expresses a neutral obligation, imposed by rules or circumstances. 〜べきだ expresses a moral judgement (“one should, it's the right thing”). べし on its own is the most solemn: it belongs to maxims and written orders, never to conversation. Note: in 来るべき, 来る is read きたる.

• 約束は守るべきだ。やくそくはまもるべきだ。Promises should be kept. (moral judgement)
• この書類は今日中に提出しなければならない。このしょるいはきょうじゅうにていしゅつしなければならない。I have to submit this document by the end of today. (practical obligation)