Practice JLPTHow to tell them apart
The key difference is the basis of your certainty. はず is a reasoned expectation based on facts, logic, or how things normally work, while に違いない is a strong personal conviction or intuitive guess. Use はず when you can explain why something should be true, like あの本は高かったはずだ because you saw the price. Use に違いない when you are making a confident inference without solid proof, like あの人は日本人に違いない from appearance alone. Swapping them can sound odd: に違いない implies gut conviction, not logical deduction.
- はず Use はず when your expectation comes from facts, logic, or how things normally work.
- に違いない・にちがいない Use に違いない when you are personally convinced of something without solid proof.
| Grammar point | Meaning | JLPT level |
|---|---|---|
| はず hazu | Expectation / should be | N4 |
| に違いない・にちがいない ni inai nichigainai | Must be / no doubt about it | N4 |
はず hazu
Expectation / should be
I expect that ~; it is expected that ~; ~ is expected to; I am fairly certain that ~; should; ought to; it is natural that ~; no wonder ~ | A dependant noun which expresses the speaker’s expectation that something will take place or took place or that someone/something is or was in some state.
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あの本は高かったはずだ。
I expect that book was expensive.
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そのアパートは奇麗なはずです。
I expect that apartment is clean.
に違いない・にちがいない ni inai nichigainai
Must be / no doubt about it
There is no doubt that ~; must be ~; no doubt | The speaker is convinced that there is no mistake on his part in guessing something.
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あの人は日本人に違いない・違いありません。
That person must be Japanese.
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あの先生の試験は難しいに違いない・違いません。
That teacher's exams must be hard.
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 — meanings, usage and example sentences — so you can tell them apart.