How to tell them apart
に決まっている expresses a confident judgment about what is naturally expected or logically bound to happen, often based on common sense or obvious reasoning, while に違いない is a strong inference or guess about something the speaker cannot directly confirm, like deducing someone's nationality from appearance. Swapping them feels off: 安く買いたいに違っている sounds like you are guessing about a hidden fact rather than stating an obvious truth, and あの人は日本人に決まっている implies it is a foregone conclusion rather than a deduction.
- に決まっている Use に決まっている when stating what is obvious or logically bound to be true by common sense.
- に違いない・にちがいない Use に違いない when you are making a strong inference or guess about something you cannot directly confirm.
| Grammar point | Meaning | JLPT level |
|---|---|---|
| に決まっている ni matteiru | Definitely / certain to be | N3 |
| に違いない・にちがいない ni inai nichigainai | certainly is; must be the case | N4 |
に決まっている ni matteiru
Definitely / certain to be
に決まっている expresses the speaker's strong conviction that something is certain to happen or be the case. It attaches to the plain form of a verb (or a verb in the plain negative), and the tone is confident and emphatic, often used in casual speech.
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そんな計画は失敗するに決まっている。
Such a plan is bound to fail.
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うちの母はノーと言うに決まっている。
My mother is certain to say no.
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.
に決まっている and に違いない・にちがいない 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.