～に違いない
must be / no doubt
～に違いない（～にちがいない）expresses strong certainty: the speaker has not confirmed the fact directly, but the evidence points clearly to one conclusion. It attaches to the plain form of a verb or い-adjective, and directly to a noun or な-adjective stem (no だ in between).
It maps closely to English “must be” or “no doubt”. Literally, 違いない means “there is no mistake (that ...)”.
Confident conclusions from evidence
Use ～に違いない when the situation lets you draw only one reasonable conclusion.
彼は犯人に違いない。
“He must be the culprit.”
- 彼（かれ）= he
- は = topic marker
- 犯人（はんにん）= culprit
- に違いない（にちがいない）= must be, no doubt
電気がついているから、誰か家にいるに違いない。
“The lights are on, so someone must be home.”
- 電気（でんき）= (electric) light
- が = subject marker
- ついている = is on
- から = because, so
- 誰か（だれか）= someone
- 家（いえ）= home
- に = location marker
- いる = to be (somewhere)
- に違いない（にちがいない）= must be
How to form it
Plain form + に違いない — nouns and な-adjectives attach directly, without だ.
- 犯人（はんにん）= culprit → 犯人に違いない（はんにんにちがいない）= must be the culprit
- 行く（いく）= to go → 行くに違いない（いくにちがいない）= must be going
- 高い（たかい）= expensive → 高いに違いない（たかいにちがいない）= must be expensive
Key idea
～に違いない is a confident guess: you have not seen the fact with your own eyes, but the evidence leaves no room for doubt in your mind.
Other N3 grammar
Updated .