How to tell them apart
Both forms express strong certainty but with different nuance. に決まっている states the speaker's firm conviction that something is obviously true or bound to happen, as in 彼は来ないに決まっている. に違いない makes a confident inference from evidence about something not directly confirmed, as in あの人は日本人に違いない.
- に決まっている Use に決まっている to state your firm conviction that something is obviously true or bound to happen.
- に違いない・にちがいない Use に違いない to make a confident inference from evidence 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 definitely true or bound to happen, often based on assumption rather than evidence. It follows the plain form of verbs and i-adjectives, as in 失敗するに決まっている, and follows nouns and na-adjective stems directly without だ or な, as in 出任せに決まっている. It sounds assertive and subjective, so avoid using it for neutral facts and be careful not to confuse it with polite decisions like に決めました.
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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 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.
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.