べき

Expresses what someone should or ought to do, based on moral judgment, duty, common sense, or what is fitting in light of a rule or situation. It states a firm opinion about what is right or proper.

Used to state firm opinions about duty, morality, or appropriate behavior, often in general statements, advice, editorials, and discussions. It sounds stronger and more judgmental than 〜たほうがいい. Compared with なければならない, which stresses obligation or necessity, べき centers on the speaker's judgment of what is right or proper, though that judgment may be grounded in rules or requirements.

• べきだN2「べきだ」 is the predicate form of 「べき」 plus 「だ」 ending a sentence, with the same meaning. When modifying a noun, only 「べき」 can be used (「読むべき本」); you cannot say 「読むべきだ本」. The past form 「べきだった」 often expresses regret.
• べきではないN3「べきではない」 is the negative form of 「べき」, meaning 'should not' — the meaning is the opposite, so they cannot be swapped. The past form 「べきではなかった」 expresses regret or blame. To say 'should not', use 「〜べきではない」, not 「〜ないべきだ」.
• てしかるべきだN1「てしかるべきだ」 attaches to the te-form of a verb (often the passive). It is more formal and indirect than 「べき」, typically used to comment on others or society, and often implies dissatisfaction with the current state. It generally does not express the speaker's own will and is rarely used to directly demand something of someone.
• なければいけない / なければならないN4「なければいけない／ならない」 emphasizes objective necessity or obligation, and is also used for personal reasons (such as having to submit a report). 「べき」 is a subjective claim based on reason. The two are interchangeable in many contexts, but 「べき」 focuses on judgment rather than necessity.
• ものだN2Only the advisory sense of 「ものだ」 is close to 「べき」: 「ものだ」 states what should be so based on social common sense, in a more general way, like an elder lecturing. 「べき」 is the speaker's subjective assertion, firmer, and often used in argumentation. 「ものだ」 also has separate senses of recollection and deep emotion.
• 〜たほうがいいN4「〜たほうがいい」 is advice for a specific situation, softer in tone and mostly used in conversation. 「べき」 makes a judgment based on reason or duty, and is stronger and more principled. The former attaches to the ta-form of a verb, with the negative being 「ないほうがいい」.

べきだ is the sentence-final form of べき, べきではない is its negative ('should not'), and てしかるべきだ is a formal way of saying something is naturally appropriate. なければならない emphasizes obligation or necessity rather than the speaker's judgment of what is proper, ものだ states what is natural or customary, and 〜たほうがいい is softer, practical advice.