べきだ

1. 1Should / ought to: states what is morally right or expected as a matter of duty or common sense.Ex. 1
2. 2Should: offers advice or a recommendation, often softened with と思います.Ex. 2
3. 3In the past form べきだった, expresses regret that something should have been done but was not.

べきだ gives the speaker's subjective judgment about what is right or proper, and it sounds firm and somewhat stiff. It is common in opinions, advice, essays and debates. When addressed directly to someone, it can sound preachy, so it is often softened with と思います. For external rules or unavoidable necessity, なければならない is more natural than べきだ.

• べきではないN3「べきではない」is the negative form of 「べきだ」, meaning 'should not'. Do not say 「〜ないべきだ」. 「べきではなかった」refers to having done something one shouldn't have, while 「べきだった」usually means failing to do something one should have.
• てしかるべきだN1「てしかるべきだ」attaches to the te-form of verbs (often passive) and is more formal and indirect than 「べきだ」, typically used when commenting on others or society, often implying dissatisfaction rather than making direct demands. When it means 'ought to', it can usually be replaced with 「べきだ」, but unlike 「べきだ」it generally does not express one's own resolve.
• ねばならないN2「ねばならない」attaches to the stem of the nai-form (する→せねば) and expresses strong obligation ('must'), with a literary flavor common in writing and speeches but rare in conversation. When expressing 'what ought to be', it is often interchangeable with 「べきだ」.
• 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／なければならない」emphasizes objective necessity or obligation arising from specific circumstances or rules, whereas 「べきだ」is the speaker's assertion based on their sense of what is right. The same situation often allows either, differing only in perspective. These attach to the nai-form.
• て当然だN2「て当然だ」attaches to the te-form and evaluates a result, reaction, or behavior as natural or deserved—even bad ones, e.g., 「体を壊して当然だ」. It also conveys 'deserve/ought to' (as in 「返して当然だ」), but its tone is evaluative; 「べきだ」makes a direct assertion.
• 〜たほうがいいN4「〜たほうがいい」attaches to the ta-form and offers lighter advice suited to a particular situation. 「べきだ」attaches to the dictionary form and asserts what should be done based on principle, sounding much stronger—and preachy if said to a superior.

べきだ states what is right in the speaker's judgment, while なければならない/ねばならない express necessity or obligation from rules or circumstances, and 〜たほうがいい is softer, practical advice. べきではない is its negative counterpart, while てしかるべきだ (formal) and て当然だ describe something as natural or deserved rather than as a duty.