～べきだ— What One Ought to Do

• 約束は守るべきだ。

  Yakusoku wa mamoru beki da.

  You should keep your promises.

  べきだ states keeping promises as the morally proper thing to do.

• 人の悪口を言うべきではない。

  Hito no waruguchi o iu beki de wa nai.

  You should not speak ill of others.

  べきではない expresses that badmouthing others is something one ought not to do.

• 学生はもっと本を読むべきだと思う。

  Gakusei wa motto hon o yomu beki da to omou.

  I think students should read more books.

  べきだ conveys a principled recommendation about what students ought to do.

• ・Register: neutral to formal; can sound preachy, so use carefully with superiors.
• ・する becomes both するべき and the more formal すべき.
• ・べきだった means 'should have (but didn't)'.

Commonly confused with

The JLPT rarely asks whether you recognise a pattern — it asks which of several similar patterns fits. These are the ones ～べきだ is tested against.

• ～ずにはいられない — Simply Cannot Resist
• ～ないではいられない — Can't Hold Back
• ～というものではない — Not Necessarily So
• ～とはかぎらない — Not Always the Case
• ないことはない — It's Not That You Can't