How 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 | 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 expresses the speaker's expectation that something is or should be the case, based on reasoning. It follows plain-form verbs and い-adjectives, な-adjectives and nouns with な, and combines with がない or もない to strongly deny that something could be true.
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そんなの、身体にとっていいはずがないじゃありませんか。
That can't be good for your health, can it?
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一週間でどれほどのことが学べるか、自分でもびっくりするはずだよ！
You'd be surprised what you can learn in a week.
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 the speaker's confident conclusion that something must be true, with no doubt about it. It follows a plain-form verb or adjective, or a noun, and is common in both casual and formal speech.
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彼女はがんにかかっていることを知ったにちがいない。
She must have known that she had a cancer.
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まさかオレがごねずにすっきり起きると思っていなかったに違いない。
Certainly she wouldn't have dreamt that I would get up promptly without grumbling.
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.