How to tell them apart
Both express certainty, but はず describes a natural expectation that should follow from a reason, plan, or how things normally work, while に違いない expresses the speaker's firm conviction that something is certainly true. For example, 彼は来るはずだ means you expect him to come because of a promise or schedule, whereas 彼は犯人に違いない means you are almost sure he is the culprit from the evidence. In short, はず is should be so and allows surprise if wrong, while に違いない is must be so with stronger assertiveness about present or past facts.
- はず Use はず to express an expectation that something should naturally be so given a reason or premise.
- に違いない・にちがいない Use に違いない to express strong conviction that something certainly is the case.
| Grammar point | Meaning | JLPT level |
|---|---|---|
| はず hazu | Speaker's natural assumption; something ought to be so | N4 |
| に違いない・にちがいない ni inai nichigainai | certainly is; must be the case | N4 |
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.
に違いない・にちがいない ni inai nichigainai
certainly is; must be the case
に違いない expresses a very strong conviction that something must be true, used when the speaker infers a conclusion from evidence or reasoning rather than stating a confirmed fact. It attaches to the plain form of verbs and i-adjectives, as in 書いたに違いない or 高い に違いない, and to nouns and na-adjective stems, as in 兄弟に違いない, with だったに違いない for past nominal predicates. It sounds assertive in speech and writing, but it is not used for promises or facts the speaker has confirmed.
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私たちは道をまちがえたにちがいない。
We must've taken the wrong road.
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シャロンはやり手にちがいない。
Sharon must be a go-getter.
はず and に違いない・にちがいない are both JLPT N4 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.