How to tell them apart
はず expresses a confident expectation based on evidence or logic — you believe something is or was the case, like 高かったはずだ (I expect it was expensive). べきだ expresses a moral or logical judgment that something ought to be done or be the case, like あるべき所 (where it should be). The key distinction is that はず describes what you think is true, while べきだ describes what you think is right or desirable. Swapping them is unnatural: saying 高かったべきだ sounds wrong because it mixes expectation with obligation, and あるはずの所 changes a normative claim into a mere guess.
- はず Use はず when you are fairly confident something is or was true based on evidence.
- べきだ Use べきだ when expressing a judgment that something ought to be done or be so.
| Grammar point | Meaning | JLPT level |
|---|---|---|
| はず hazu | Speaker's natural assumption; something ought to be so | N4 |
| べきだ bekida | is supposed to; is morally right to do | N3 |
はず hazu
Speaker's natural assumption; something ought to be so
はずだ expresses a speaker's confident expectation based on reason or evidence that something ought to be true, as in 来るはずだ or 静かなはずだ. It follows the plain form of verbs and i-adjectives, with na-adjectives taking な and nouns taking の, as in 学生のはずだ. The time of the expected situation is shown by the form before はず, while はず itself inflects for the speaker's viewpoint, such as はずだった for a past expectation and はずがない for strong denial.
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バスはすぐ来るはずだよ。
The bus should be coming soon.
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アンがピアノをひいているはずがない。
Ann cannot be playing the piano.
べきだ bekida
is supposed to; is morally right to do
べきだ expresses that an action is the right or expected thing to do, based on duty, morals, or common sense. It attaches to the dictionary form of a verb, as in 行くべきだ or 食べるべきだ, with する becoming すべきだ. Forms include べきだった for past obligation, べきではない for negative obligation, and べきである in formal writing. It sounds strong and judgmental, so it is common in rules and criticism but can sound harsh as casual advice.
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君自らが会議に出席すべきであったのだ。
You should have attended the meeting in person.
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君は彼の仕事を手伝うべきだった。
You should have helped him with his work.
はず and べきだ are JLPT N4–N3 grammar points that Japanese learners often mix up. This page compares them side by side on meanings, usage and example sentences, so you can tell them apart.