～べき
should / ought to
～べき expresses a strong recommendation or a moral obligation: this is what one should or ought to do. It attaches to the dictionary form of a verb, and the verb する has the special short form すべき (するべき is also used).
It is stronger than soft advice like ～たほうがいい: ～べきだ presents the action as the right and proper thing to do.
1. Saying what should be done
Use dictionary form + べきだ to state a duty or a firm recommendation. The negative is ～べきではない (“should not”).
約束は守るべきだ。
“You should keep your promises.”
- 約束（やくそく）= promise
- は = topic marker
- 守るべきだ（まもるべきだ）= should keep
2. Regret with べきだった
The past form ～べきだった means “should have done” — the right action was not taken, and the speaker regrets it.
もっと早く医者に行くべきだった。
“I should have gone to the doctor sooner.”
- もっと = more
- 早く（はやく）= early, soon (adverb)
- 医者（いしゃ）= doctor
- に = destination marker
- 行くべきだった（いくべきだった）= should have gone
Notice that the verb before べき stays in the dictionary form; only the final だ changes to だった.
How to form it
Verb dictionary form + べき（だ）; する becomes すべき
- 守る（まもる）= to keep → 守るべきだ（まもるべきだ）= should keep
- 行く（いく）= to go → 行くべきだ（いくべきだ）= should go
- する = to do → すべきだ = should do
Key idea
～べき marks an action as the right thing to do. Use it for duties and firm recommendations, and ～べきだった when the right thing was, regrettably, not done.
Other N3 grammar
Updated .