～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.