〜に違いない
Must be, without a doubt (conviction)machine translation
FormationPlain form + に違いない (nouns・な-adjectives attach directly, without だ)
〜に違いない expresses the speaker's near-certain conviction: 電気がついているから、彼は家にいるに違いない (the lights are on, so he must be home). Based on evidence at hand or strong intuition, the speaker concludes "it can't be otherwise."
Connection: plain form + に違いない; nouns and な-adjectives attach directly, without だ: 犯人はあの男に違いない. The polite form is に違いありません. This pattern leans toward inner monologue, writing, and detective fiction — in everyday conversation Japanese speakers more often use きっと〜だろう.
On the certainty scale you've been building: かもしれない (a mere possibility) < だろう (probably) < に違いない (firm conviction). Compared with はずだ — a RATIONAL inference from clear grounds such as schedules or promises — に違いない leans toward the speaker's own EMOTIONAL certainty. At the same level of certainty, N3 also has 〜に決まっている, which is more colloquial and blunt.
Examples
電気がついているから、彼は家にいるに違いない。
The lights are on, so he must be home.
この字は田中さんが書いたに違いありません。
This handwriting must be Tanaka's.
犯人はあの男に違いない。
The culprit must be that man.