Unit 5 - Section 2: 〜に違いない
〜に違いない (must be / no doubt)
に違いない is a strong inference: you are sure from the evidence, not reporting a fact you saw.
あの人は日本人に違いない。
That person must be Japanese. (inference)
鍵を家に忘れたに違いない。
I must have left my keys at home.
Weaker cousins: はずだ (expected from reason), だろう (probably). Stronger written twin: に相違ない.