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〜に違いない

must be, surely

Plain form + に違いない (な-adj, Noun: no だ)

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に違いない expresses a strong conviction: あの人は日本人に違いない, "that person must be Japanese". Literally, "there's no mistake that".

It follows a plain form; nouns and な-adjectives attach directly, without だ. The polite form is に違いありません.

It's based on the speaker's intuition or impression, which makes it a little more emotional and subjective than はずだ, which reasons from facts or expectations. Compare the scale: かもしれない (maybe) → だろう (probably) → はずだ (should be) → に違いない (must be).

It's more common in writing and narration than in casual speech, where きっと〜と思う or 絶対 often does the same job.

Examples

  • あの人ひとは日本人にほんじんに違ちがいない。

    That person must be Japanese.

  • 電気でんきが消きえている。もう寝ねたに違ちがいない。

    The lights are off. They must have gone to bed.

  • 犯人はんにんはこの中なかにいるに違ちがいない。

    The culprit must be one of these people.

  • こんなにおいしいのだから、高たかいに違ちがいない。

    It's this delicious, so it must be expensive.

  • 彼女かのじょは今いまごろ、心配しんぱいしているに違ちがいありません。

    She must be worried by now.

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